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\title{THE WELL-DESIGNED CHILD}
\author{John McCarthy, Stanford University}
\date{\jmcdate ,\ \theTime}
\maketitle
% /u/jmc/e08/perlis
\abstract{This article is inspired by recent psychological studies
confirming that a child is not born a \emph{blank slate} but has
important innate capabilities. An important part of the
``learning'' required to deal with the three dimensional world of
objects, processes, and other beings was done by evolution. Each
child need not do this learning itself.
By the 1950s there were already proposals to advance artificial
intelligence by building a child machine that would learn from
experience just as a human child does. What innate knowledge the
child machine should be equipped with was ignored. I suppose the
child machine was supposed to be a blank slate.
Whatever innate knowledge a human baby may possess, we are
interested in a \emph{well-designed} that has all we can give it.
To some extent, this paper is an exercise in wishful thinking.
The innate mental structure that equips a child to interact
succesfully with the world includes more than the {\em universal
grammar} of linguistic syntax postulated by Noam Chomsky. The
world itself has structures, and nature has evolved brains with ways
of recognizing them and representing information about them. For
example, objects continue to exist when not being perceived, and
children (and dogs) are very likely ``designed'' to interpret
sensory inputs in terms of such persistent objects. Moreover,
objects usually move continuously, passing through intermediate
points, and perceiving motion that way may also be innate. What a
child learns about the world is based on its innate mental
structure.
This article concerns {\em designing} adequate mental structures
including a {\em language of thought}. This {\em designer stance}\ %dennett
applies to designing robots, but we also hope it will help
understand universal human mental structures. We consider what
structures would be useful and how the innateness of a few of the
structures might be tested experimentally in humans and animals.}
In the course of its existence we'll want our robot child to change.
Some of the changes will be be development, others learning.
However, this article mainly takes a static view, because we don't
know how to treat growth and development and can do only a little with
learning.
\newpage
% Empiricists vs. nativists
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\tableofcontents
\newpage
\section{Introduction}
\label{sec:intro}
% ``blooming, buzzing confusion'' is due to William James
% googling it gets articles on categorization, monadic
Ren\'e Descartes proposed that a philosopher should assume as little
about the world as possible and gradually build reliable knowledge
using step-by-step reasoning, observation and experiment. John Locke
proposed that a baby starts out as a ``blank slate''. Bertrand
Russell \cite{Russell13} proposed starting with sensation and building
up a theory of the world on that foundation. Positivist philosophy
and behaviorism in psychology advocated the same
methodology.\footnote{The earlier web version of this paper mistakenly
ascribed the blank slate doctrine to Descartes.}
Likewise, the AI learning literature is based on learning to recognize
patterns in the inputs to a machine or computer program. A baby that
started with its sensations and built a world-model from that might be
called a \emph{Lockean baby}. I don't know whether any computer
program starting from sensation has ever learned the existence of
semi-permanent physical objects that persist even when not perceived.
For a philosopher, starting from sensation and building up from there
has the advantage of avoiding \emph{a priori} assumptions, but neither
actual science nor common sense works that way. Instead there is
almost always a complex structure of ideas that is modified piecemeal.
%Positivism in philosophy and behaviorism in psychology are pretty well
%superseded.
Evolution solved a different problem than that of starting a baby
with no \emph{a priori} assumptions.
Instead of building babies as Lockean philosophers taking nothing
but their sensations for granted, evolution produced babies with
innate prejudices that correspond to facts about the world and babies'
positions in it. Learning starts from these
prejudices.\footnote{There
is a complication. Appropriate experience is often
required for the genetically determined structures to develop
properly, and much of this experience can be regarded as learning.}
Evolution isn't perfect and human babies don't have all useful prejudices.
What is the world like, and what are these instinctive
prejudices?\footnote{I don't argue that a Lockean baby wouldn't work
at all. Only that it would have a much longer babyhood than human
babies do. Even \emph{``universal grammar''} might be learned from
experience. I just think evolution has learned to build in many of
these features. Therefore, it is an empirical question whether a
particular ability is learned or innate.}
This paper studies the problem as follows.
\begin{itemize}
\item We ask what the world is like at the level at which people and
robots interact with it. Particularly important is what we call
\emph{the common sense informatic situation}. It relates the partial
information about the world that can be obtained and the kinds of
results that can be achieved in the world with these
actions.\footnote{We emphasize the effect of the actions on the
world and not the new sensations that result from the action.}
\item Next we ask what knowledge would be useful to build into a robot
or for nature to have built into babies. We do this without regard
to which feature are actually present.
\item Now we ask what features seem to be present in babies.
\item Finally we consider what experiments have been made and can be
made to discover what innate knowledge nature has given us.
% See
% \cite{Spelke94} for some experimental results.
\end{itemize}
In so far as we have an idea what innate knowledge of the world would
be useful, AI can work on putting it into robots, and cognitive
science and philosophy can look for evidence of how much of it evolved
in humans. This is the {\em designer stance}.\footnote{Designer
stance is related to Daniel Dennett's \textsf{design stance}
\cite{Dennett78}, but Aaron Sloman has persuaded me that I was not
using it quite in the way Dennett used design stance.
1999 note: Dennett has approved this usage of \emph{design stance},
but I still want a distinct term.
}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{What the World is Like}
\label{sec:world}
The most straightforward philosophical way of thinking about the
world's interaction with a baby or other person is in terms of its
input-output relations with its environment. Unfortunately for our
philosophical convenience, but fortunately for our survival, this is
not the way the world is structured.
The world's structure is not directly describable in terms of the
input-output relations of a person. The basic structure of the world
involves the interaction of elementary particles on time scales of
$10^{-25}$ seconds, but intelligence did not evolve in structures of
small numbers (mere billions) of elementary particles. When
intelligence evolved, it was in structures of the order of $10^{26}$
elementary particles and time scales of the order of $10^{-1}$ seconds
to years and very complex hierarchical structures. Even then only
some of the higher level and slower objects and events are directly
perceivable. Even bacteria, weighing one picogram $10^{-12}$ grams,
have about $10^{10}$ atoms. The mass of a small virus is about 10
attograms [attogram = $10^{-18}$ grams].
Even on the human size and time scale, the world is not
structured in terms of human input-output relations. Moreover, much
of the determinism of the world at the microscopic level appears as
non-deterministic at the level at which a person can interact with the
world. \footnote{Whether a baseball will pass over the plate is
approximately deterministic once it leaves the pitcher's hand, but
the batter and the spectators have to guess.}
Animal behavior, including human intelligence, evolved to survive and
succeed in this complex, partially observable and very slightly
controllable world. The main features of this world have existed for
several billion years and should not have to be learned anew by each
person or animal. In order to understand how a well-designed baby
should work, we need to understand what the world is like at the gross
level of human interaction with the environment.
Here are some of the world's characteristics. A baby innately
equipped to deal with them will outperform a Lockean baby.
\begin{description}
\item[appearance and reality] Some properties of the world are stable
even though their appearances change. Objects last from seconds to
centuries, while appearances change in fractions of a second.
Therefore, humans, animals and robots are better off representing
information about objects in so far as it can be obtained by
observation and inferred from past experience or is
innate.\footnote{Kant distinguished between appearance and reality,
but AI and psychology need to study the distinction at a more
mundane level than Kantian philosophers have brought themselves to
do. I don't believe the study of platonic or neo-platonic forms
will help understand the relation between physical dogs and the
various ways they could affect the senses of a child or robot.}
\cite{McC99c} presents a puzzle in which the subject must experiment
to determine the 3-d reality behind the 2-d appearances.
\item[things of interest] Some aspects of the world are relevant to an
animal's or person's survival or prosperity, and others are not.
However, notice that human and animal curiosity concerns many
aspects of the world not related to survival or enjoyment.
Other details of shape and pattern are not interesting.
\item[semi-permanent objects] Much of the world consists of
three-dimensional objects that have masses, moments, compliances,
hardnesses, chemical composition, shapes, outer surfaces with
textures and colors, are often made of identifiable parts and which
move relative to the rest of the object. A particular object can
disappear from perception and reappear again. The location of an
object in the world is more persistent than its location in the
visual field.
Objects usually have internal structures that are not apparent to
human senses.
A baby seems to have an innate interest in the names of things quite
apart from what may be immediately useful. Thinking of it
linguistically, it is an interest in semantics, not just in syntax.
We'd better build that into our robotic children.%needs to fit other
%mention of names
\item[continuity of motion] Objects move continuously passing through
intermediate points and intermediate orientations.
\label{page:continuous1}
\item[continuous processes] Besides moving objects, there are many
continuous processes with intermediate states.
\item[two dimensional world] Because of gravity, much of the world is
two dimensional with its simpler topology. Paths can block other paths.
\item[specific objects] The environmment of a child contains other
people, usually including a mother, and parts of people including
parts of the child itself. Objects often have parts which are objects.
However, often only some of the parts are separately identifiable.
The boundaries of the parts are often not definite.
\item[solidity] Objects that are solid do not ordinarily penetrate one
another. Some are rigid and some are flexible.
\item[gravity] Objects require supporting surfaces, and an unsupported
object falls to a lower surface.
\item [kinds of objects] Objects have kinds, and objects of the same
kind have properties associated with the kind.\footnote{It might be
more parsimonious intellectually to have just a relation of
similarity between objects. However, the world as it is justifies
the bolder attitude that there are kinds, and we should build this
into our robots and expect it in our children. The use of nouns
in language presupposes more than just similarity relations.}
Babies are ready very early to learn what kinds there are.
\item[relations] Objects not only have individual properties and
belong to kinds, but different objects and kinds have relations with
one another. At least some ternary relations such as betweenness
are basic. Also ``A is to B as C is to D'' seems to be basic. In
its numerical use, it reduces to the equality of two fractions, but
the quaternary relation seems to be basic in common sense usage.
In philosophy, AI, and computer science, there is an overemphasis
on unary relations, i.e. properties.
% the unary prejudice
\item[natural kinds] Many of the objects a child encounters, e.g.
lemons, belong to {\em natural kinds}. The objects of a natural
kind have yet undiscovered properties in common. Therefore, a
natural kind is not usually definable by an \emph{if-and-only-if}
sentence formulated in terms of observables.
\item[fundamental kinds] Animate objects are to be understood in terms
of their desires and actions. Inanimate objects are passive.
Some objects are edible by humans and some are not. These kinds
pervade the baby's environment.
\item[abstractions] Kinds belong to higher kinds and have relations.
Red is a color and color is a quality. This is a fact of logic
rather than about the physical world, but its usefulness is
dependent on objects being naturally grouped into kinds rather than
being all completely different.
\item[sets and numbers] There are sets of objects and other entities.
This includes both sets of objects perceivable on a single occasion
and sets organized more abstractly. Sets can often be counted.
Some are more numerous than others and this is significant. Sets
can be used up, e.g. all the food can be eaten.
\item[situations and kinds of situations] Kinds of situations recur.
\item[the body] The baby itself and its parts are objects.
\item[movability] Some objects can be moved with the arms and legs of
a child.
\item[responses] Mothers help a baby that cries.
\item[love] A mother loves her baby.
\item[unimportant aspects] Many aspects of the world are ordinarily
unimportant for a human or animal. For many purposes, shadows are
mere epiphenomena.
\item[quantitative physics] Humans could act more precisely if our
senses gave us numerical measures of time, distance, velocity,
humidity, temperature, etc.; our minds could do rapid arithmetic
with them, and we could give numerical values to the signals telling
our muscles how fast to contract. Nature didn't give us this, but
we can build it into our robots as an add-on to the kinds of
semi-quantitative information human senses give us.
\item[Newtonian physics] While the world is not fully determinate at
the level at which humans interact with it, many events are related
in a simple numerical way. For example, $s = {1\over 2}gt^2$
describes the distance a body will fall, and hot bodies cool at a
rate proportional to the difference in temperature between a body
and its surroundings.
\item[atoms] The material world is built up from atoms and molecules.
It is more fundamental than most of the above facts but is similar
to them. While even ancient Greek philosophers like Democritus
could conjecture that the world was built from atoms, John Dalton
was the first person to offer scientific evidence for the fact.
\item[mathematics] Very complex structures, (e.g. groups, rings and
fields), exist in a mathematical sense.
\item[mathematics of the world] Very complex mathematics is
``unreasonably effective'' in understanding and controlling the
physical world.
\
\end{description}
All the above are facts about the world. All but the last few may or
may not be represented innately. We can also imagine that we might have
evolved innate knowledge of the above mathematically expressible
facts, but alas we didn't. The items listed are certainly not a
complete set of facts about the commonsense world that a well-designed
child might know about. Moreover, innate mechanisms for dealing with
phenomena related to these facts do not always take a form describable
as having certain knowledge.
In the next section we consider which of the above facts a child might
know about or have special mechanisms for dealing with.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Human Mental Characteristics}
\label{sec:human}
Here are some human mental characteristics that affect what abilities
might be innate.
\begin{description}
\item[evolved from animals] The human was not designed from scratch.
All our capabilities are elaborations of those present in in
animals. Daniel Dennett \cite{Dennett78} discusses this in the
article ``Can a Computer Feel Pain''. Human pain is a far more
complex phenomenon than an inventor would design or a philosopher
intuit by introspection. As Dennett describes, kinds of pain are
associated with levels of organization, e.g. some are in the
structures we share with reptiles.\footnote{Here the facts of
evolution have an observable payoff akin to Haeckel's ``Ontogeny
recapitulates phylogeny''.}
\item[distributed mechanisms] We are descended from animals that
mostly have separate neural mechanisms controlling separate aspects
of their lives. We have these separate mechanisms too but are more
capable than animals of observing their state and integrating their
effects.
\item[central decision making] A mobile animal can go in only one direction
at a time. Therefore, animals above a certain level, including all
vertebrates, have central mechanisms for making certain decisions.
Very likely sponges don't need a central mechanism.
\item[little short term memory] Compared to computers, humans have very
little short term memory. In writing a computer program it is
difficult to restrict oneself to a short term memory of $7\pm 2$
items.
\item[slowness] Human performance is limited by how slowly we process
information. If we could process it faster we could do better, and
people who think faster than others have advantages. For this
reason we need to perceive states of motion and not merely
snapshots. Computer programs often work with snapshots, but even
they suffer from slowness when they don't represent states of motion
directly.
\item[incompleteness of appearance] When a person looks at a scene,
only part of the information available seems to go all the way in.
There is the blind spot, but there is more incompleteness than that.
What seems subjectively to be a complete picture really isn't. The
picture has to be smoothed over in such a way that a detailed look
at a part of it sees no inconsistency. While the phenomenon is most
obvious for vision, it surely exists for the other senses as well.
\item[memories of appearance] I suppose this opinion will be
controversial among psychologists and neurophysiologists, but I
state it anyway. What humans remember about the appearance of an
object are attached to their more stable memories of its physical
structure and maybe even to memories of its function. For example,
my pocket knife is in my pocket, and I remember what blades it has.
If required to draw it from memory, I would consult this memory of
its structure and draw that. Only a small part of the information
used would be visual memories. Physical structure is more stable
than appearance.
\item[curiosity] Humans and animals are curious about the world. Just
how curiosity is focussed isn't obvious.
\item[supposed to do] It is often asserted that children learn what to
do in situations by being rewarded. The innate mechanism may be
more powerful than that.
Children and adults have a concept that in a particular kind of
situation there are actions ``that one is supposed to do''. One
learns what one is supposed to do and does it without reinforcement
of the specific kind of response. Example: I told several people,
``See you later,'' and an 18 month old baby whom I was not
specifically addressing said, ``Bye-bye''. Children who try to
learn what they are supposed to do in a situation and do it will
survive better than those who need to learn responses by
reinforcement. The race was reinforced---or maybe it was our
mammalian ancestors.
\item[senses] The characteristics of human senses are an accidental
consequence of our evolution and our individual development. A
blind person lives in the same world of objects as a sighted person.
It is just that sighted persons have an advantage in learning about
them. A person with an infra-red detecting pit in his forehead like
a pit viper (or some computer terminals) would have a further
advantage in distinguishing people, warm-blooded animals and stoves.
A person with a bat-like sonar might ``see'' internal surfaces of
itself and other people.
This is not the best of all possible worlds---only a pretty good one.
\end{description}
It would be interesting to look more closely at how human mental
characteristics differ from those of animals.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{What Abilities Could Usefully be Innate?}
\label{sec:innate}
Taking into account what the world is like and what our nervous
systems are like, what knowledge and abilities are possibly and
usefully innate? Many of them correspond to the facts about the
world discussed in section \ref{sec:world}.
\begin{description}
\item[some objects persist even while not sensed] Having this
prejudice is fundamental to the survival of humans---and probably to
other land vertebrates. A dog chasing a ball will look for it if it
disappears behind something.
\item[identify object] Identify a part of the current stimulus pattern
as coming from an object. Remember aspects of the object as the
same as a previous object or as coming from a new object. The task
is basically the same whether the stimuli are visual, tactile,
auditory or olfactory or a combination. Success involves
recognizing repeated instances of the same object or the same kind
of object. Present machine learning schemes are more suited to
recognizing kinds of objects than for recognizing individual
objects. Both are needed.
What innate structures are suited for this? At least some of these
structures are independent of the sensory modality.
\item[natural kinds] The child is predisposed to name kinds of
entities and to expect that the objects of a kind that is recognized
by superficial properties will have additional properties in common.
For example, adults call some objects lemons, and all lemons turn
out to have similar taste and to have similar seeds.
\item[three-dimensional objects] The world contains three-dimensional
objects, and humans know about them. While non-blind people usually
get most of their information about objects from seeing them, what
we know about objects should not be regarded as a collection of 2-d
pictures. The objects are far more stable than pictures of them can
be, because they are seen at a variety of angles and lighting
conditions. We learn about objects from 2-d pictures, but they are
not constructs from 2-d pictures.
Advocates of an initial {\em tabula rasa} have proposed that a baby
{\em learns} that its sensations should be organized around external
objects. Maybe a mechanism for learning this could exist, but a
baby would learn faster if this much were innate. In fact
animal thought also seems to presume external objects. Many
specific instincts, e.g. related to hunting, presuppose them.
A baby also has no difficulty with two dimensional representations
of three dimensional objects. A baby apparently doesn't have to
be taught that a picture of a dog in a book represents some real dog.
\item[objects have colors] Our visual system goes to a lot of trouble
to ascribe colors to objects in ways that are independent of
lighting. When this fails, we notice it.
\item[expect an object to have a location] Since a physical object a
person has perceived ordinarily continues to have a location even
when it is no longer perceived, because it or the person has moved,
it is advantageous for the person to expect it to have a location.
He might want to look for it or reason about its effects on other
objects, e.g. as described in \cite{Spelke94}.
\item[perceive motion as continuous] Although our visual perceptions
of objects are discrete because of our saccadic movements, we
perceive objects as moving continuously. We evolved to interpret
our sense data, and not just visual sense data, in terms of
continuous motion. Perceiving motion as continuous may have evolved
very early among vertebrates. I suppose this involves an
approximate differentiation of the position.
\item[recognize parts] Recognize parts of an object and their relations
to the others. It would be interesting if there were an ability
to recognize certain physical structures, e.g. towers and walls,
analogous to the ability to recognize a grammatical sentence.
\item[kind of situation] Identify the current situation as being of a
certain kind.
\item[focussed curiosity] In the Shannon quantitative measure of
information, there is just as much information to be obtained from
the pattern of saw marks on the boards of my office wall as there is
about what is available for lunch or what can be obtained by
research on artificial intelligence. Curiosity needs to be focussed
on what is potentially relevant to the baby or robot. Notice that
human curiosity, as it ought to be, is quite broad---but it is also
selective. Part of the answer is that curiosity is focussed on
getting more information about kinds of object that have been
identified.
\item[noise rejection] Certain appearances are usually noise,
e.g. shadows. The child may be predisposed to regard shadows as
noise, i.e. to regard an object as continuing through a shadow and
to ignore the edges of shadows. Elizabeth Spelke \cite{Spelke94}
considers the recognition of shadows to be non-innate.
\item[grammar of goal regression] The recognition that a goal is
achievable because it is either already achieved or all the
preconditions of an action that achieves it are achievable. This
can be regarded as the grammar of a specific language GR, but unlike
the grammar of a spoken languages, the grammar of GR is
universal.\footnote{This may be worth a small pound on the table.
It would seem that a person, and maybe even some animals, can test
whether a goal is achievable by parsing the goal regression
structure. Of course, there are limits in how big a structure can
be parsed, but the {\em competence} puts no limit on the size. It
may be that goal-regression memory is in addition to other short
term memory and can only be used in connection with remembering
goals.}
% ask Jensen about goal regression memory.
% regression by introspection?
\item[principle of mediocrity] The baby is like other people. It can
learn about its own capabilities from observing others, and it can
learn about others by putting itself in their places. \footnote{
Astronomers use ``principle of mediocrity'' for the hypothesis
that there is nothing special on the average about our own part of
the universe or about our own point in time.}
\item[introspection] Recent work in psychology, \cite{Flavell99a} and
\cite{Flavell2000}, shows
that children develop some introspective ability by age 3, and this
ability improves with age. \cite{McC96} discusses the introspective
abilities required by a robot.
\item[pointer effect] When one uses a pointer, e.g. a pencil, to
explore or manipulate in a container, one's senses refer to the
end of the pointer and not to one's hands. This seems to be innate,
but is not a feature of helpless young babies. Maybe there's a
standard name for what I've called ``pointer effect''.
% not proprioception
\end{description}
It would be interesting if there were innate non-linguistic human
mental abilities that are not present in animals. Nothing appears
obvious, but maybe the innate part of human number sense is
qualitatively different from that of animals.
Some abilities require early experience to acquire. For example,
people blind from birth who gain sight as adults don't acquire an
image processing system fully adapted to the world as it is. However,
there is no reason to expect that they could acquire an image
processing system adapted to a quite different visual world. If this
is so, then the image processing system is still basically innate.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Features of a Language of Thought}
\label{sec:lot}
Cognitive scientists argue about whether there is a {\em language of
thought}, but its advocates haven't told us much about what it is
like. Stephen Pinker, an advocate, only tells us in
\cite{Pinker:LI94}
\begin{quote}
The hypothetical ``language of thought'', or representation of
concepts and propositions in the brain in which ideas, including
the meanings of words and sentences, are represented.
\end{quote}
A language of thought that might be used for robots or looked for in
humans is constrained by the characteristics of the baby's world and
the characteristics of the non-linguistic parts of the baby's
mind---including its limitations.
Here are some ideas about mentalese.
\begin{description}
\item[grammar is secondary] While most linguistic studies have
focussed on grammar, meaning is more important---in studying spoken
language, in proposing a language of thought and in designing
robots. A child's first speech consists of words which are attached
to things, or to appearances of things, or to sometimes ambiguous
combinations of things and appearances. ``Doggie'' is
stimulated by the sight of a dog, a picture, an animal on TV, the
sound of barking and conversation about dogs.
\item[maybe language starts with naming] A human child starts language
learning with names for objects. This desire is independent of
having any goal concerning the object. We have the option of
designing an artificial child to know a lot of language, e.g.
English and/or a logical language from the beginning. Different
experimenters will explore different approaches.
\item[parallel information] Images are presumably represented in
parallel. There is nowhere anything like a television signal
processor that handles a picture serially and repeatedly spreads it
out. This is obvious for pictures but surely applies to a lot of
other kinds of information. On the other hand, our inability to
think completely in parallel shows that many higher mental
functions are done serially.
\item[logic] For a robot, a logical language \footnote{Logicians do
not consider logic itself to be a language, but rather consider a
language to be defined by the predicate and function symbols that
are used with the logic. This is a valuable distinction and AI
and cognitive science researchers should maintain it.} will be
most suitable, but some appropriate ascriptions of beliefs and
intentions to robots will refer to information represented
non-logically. Humans \emph{probably} don't use quantificational
logic at the pre-verbal level, although we can use it when we have
to, and formal logic is often helpful when the information is
mathematical. Here's why I only say \emph{probably.} Consider the
sentence ``For every boy there's a girl who loves only him.'' Its
predicate calculus representation,
$(\forall b)((\exists g)(Loves(g,b) \land (\forall b')(Loves(g,b')
\rightarrow b' = b))$,
has three embedded quantifiers. We then ask the question, ``What can
you say about the total number of boys and girls?'' A fair number of
people uneducated in logic find the correct answer that there must be
at least as many girls as boys. I haven't done the experiment
thoroughly, but the results suggest that the sentence with the three
levels of quantifiers is understandable by many logically uneducated
people and therefore its content is somehow internally represented.
Let's design it into our child.
\item[a word at a time] Sentences uttered by humans are usually not
preformed in entirety before being uttered. A human starts a
sentence and thinks how to continue and finish it as he continues
talking. (The obvious argument is from introspection, but I suppose
experiments would confirm it.) Humans can preform sentences with
some effort. Vladimir Bukovsky tells about having composed a whole
book in prison while denied paper.
\item[chemical state] Suppose a person is hungry---a condition humans
share with dogs. This can perfectly well be only be represented by
the chemical state of the blood stream. There is no reason to have
anything like the sentence, ``I am hungry'' anywhere in the brain
until the fact has to be communicated. Similarly we don't need
anything like a sentence in the memory of a computer to represent
the voltage of its battery.
\item[virtual sentences] We may regard information that is directly
represented by the chemical state of the bloodstream or by a voltage
as expressed in {\em virtual sentences\/} along the lines of
\cite{McC79a} or \cite{Newell:KL}. We may then sometimes be able to
explain some actions as involving logical inference involving the
virtual sentences.
\item[immediate reference] Thinking about an object before one's eyes
does not require that it have a name. Something like a pointer to a
structure will do as well. We can see this, because when we have to
mention an object in speech we have to think of a name that will
enable the hearer to establish his own pointer to his mental
structure representing the object in question. Purely internal
symbolic names as in Fodor's proposed \emph{language of thought} may
be useful even if they aren't communicable.
\item[short thoughts] Thoughts are not like long sentences, although
a long sentence may be required to express a thought to another
person because of a need to translate internal pointers into
descriptions.
\item[communication] When the fact of hunger or low battery voltage has to be
communicated something like a sentence is needed. Let's call it a
pseudo-sentence until we find out more. However, a pseudo-sentence
isn't needed to stimulate eating. It also isn't necessary to
represent the rule, ``if hungry, then eat''. In view of evolution,
one would expect the fact of being hungry to be represented both
chemically and in the language of thought.
\item[future] There are other uses besides communication for
sentence-like forms. Very likely, the expectation of being hungry
by dinner time needs something different from a substance in the
blood for its representation.
\item[reasoning] The language of thought is used for reasoning.
\item[not like spoken languages] English and other spoken languages
won't do as languages of thought. Here are some reasons.
\begin{itemize}
\item Much mental information is represented in parallel and is
processed in parallel.
\item Reference to states of the senses and the body has to be done
by something like pointers. Natural languages use descriptions,
because one person can't give another a pointer to his visual
cortex.\footnote{A robot might tell another robot, ``Look through
my eyes, and you'll see it.}
\item We don't think in terms of long sentences.
\item Much human thought is contiguous with the thought of the
animals from which we evolved.
\item For robots, logic is appropriate, but a robot internal
language may also include pointers.
\item A language of thought must operate on a shorter time scale
than speech does. A batter needs to do at least some thinking
about a pitched ball, and a fielder often needs to do quite a bit
of thinking about where to throw the ball. Pointers to processes
while they are operating may be important elements of its
sentences.
\end{itemize}
I think there are additional reasons, but I haven't been able to
formulate them.
\end{description}
The language of thought may undergo major reorganizations. This may
be one reason why there is so little memory of early life. Almost
no-one can remember nursing or drinking from a baby bottle.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Experimental Possibilities}
\label{sec:experiments}
\subsection{AI}
\label{sec:ai}
There are two kinds of AI experimental possibilities. The first is to
use the ideas of this article to try to break AI systems intended to
deal with the common sense world that lack some of the capabilities
discussed in this article. The second is to use the ideas to build an
AI system. Since the ideas are not advertised as complete enough to
serve as a design, the first option seems more fun to pursue.
%Sequence extrapolators that attempt to predict the sensory future from
%the past are likely to be particularly fragile in a world of material
%objects.
\subsection{Psychological Experiments}
\label{sec:psycho}
Elizabeth Spelke \cite{Spelke94} describes a number of experiments
that she and others have done to discover and verify innate mental
abilities. The basic technique uses the fact that a baby will look
longer at something surprising than at something that seems familiar.
Here's one that was first done in 1973 \cite{Ball1973} and was repeated by
Spelke in 1993. There are experimental babies and control babies and
the experiment has two phases. In the first phase the control babies
are shown nothing. The experimental babies see an object go behind a
screen and shortly another object emerges on the other side of the
screen. The timing is such as would be appropriate if the first
object struck the second object and knocked it from behind the screen.
The babies are shown the phenomenon enough times to get bored with it
and stop paying attention.
In the second phase of the experiment the screen is removed. There
are two variants. In the first variant, the first object strikes the
second and knocks it onward. In the second variant the first object
stops short of the second, but the second object takes off as though
it had been struck. The control babies look at both variants for the
same amount of time. The experimental babies look longer at the
second variant.
The conclusion is that the experimental babies inferred that the first
object had struck the second when the event occurred behind the
screen. When the screen was removed, they were not surprised when the
expected event was shown to occur but were surprised and looked longer
when this expectation was not met.
The conclusion is that babies have innate expectations about dynamics,
i.e. are well-designed in that respect.
For details see \cite{Spelke94}.
That was an actual experiment. Now consider some possible experiments.
Suppose we want to determine whether some abilities concerned with a
specific fact about how the world is organized is innate. We compare
a baby's ability to use this fact compared to its ability to learn a
fact about an environment constructed differently from our world but
logically no more complex.
Here are some possibilities. Since I am rather innocent of the
psychological literature some of them may already have been tried.
\begin{description}
\item[three-dimensional objects] I'm skeptical that a person's notion
of a physical object is fundamentally visual. Here's an informal
experiment I actually did. The subjects attempted to draw a
statuette in a paper bag. They could put their hands into the bag
and feel it as much as they wanted to. The quality of a subject's
drawing, except for surface colors, was similar to what that subject
would have produced looking at the object except in one case. The
object was a statuette of an owl, and the subject who misperceived
it as an angel produced an inferior drawing.
It would be worthwhile to use this and analogous techniques to
explore people's concepts of three-dimensional objects. I would
think that it is possible to investigate how babies perceive objects
they are only allowed to touch and then see. The baby could feel an
object in a paper bag and then see either the same object or a
different object. The hypothesis is that the subject would regard
seeing the same object as less surprising than seeing a different
object.
\item[anticipating the future] To eat when hungry doesn't require
having in mind anything like a sentence. However, to know that one
will be hungry 4 hours from now may require it. Maybe this is where
humans and apes part company. Can an ape that is not hungry perform
a non-habitual action, e.g. putting a key by an empty food box, in
anticipation of being hungry later?
\item[unethical experiment] A Lockean baby would do as well in
flatland as in our space. Imagine arranging that all a baby ever
sees is a plan of a two-dimensional room and all his actions move
around in the room. Maybe the experiment can be modified to be safe
and still be informative.
\item[continuity of motion] The Lockean baby is brought up in an
environment in which motion is discrete. Imagine that the baby's
world is a Macintosh screen. Objects move without passing through
intermediate points. The baby moves an object by clicking on the
initial and final locations. The experiment is to determine how
well a baby will do in such a world. This one might be tried with
an animal.
\item[attention experiment] If a baby is built to expect objects to
behave as solids, then it will be surprised when objects appear to
interpenetrate. It might pay longer attention to such a scene.
\item[inconsistencies] Babies might or might not find Escher-type
drawings surprising.
\item[geographical representation] Consider a maze with a glass top.
Does it help an animal find food if it can walk around on the top of
the maze before entering it? The top could have small holes that
the smell of the food could get through. One psychologist opined
that dogs would be helped and rats would not.
The experiment would test whether the animal can represent a scene
by something like an image.
\item[goal regression in animals] An animal seeks a goal but discovers
that a precondition must be achieved first and undertakes to do it.
Then it discovers a precondition for the precondition, etc. Suppose
the animal has been trained to achieve B as a precondition for
achieving when A isn't already true. It has been trained to achieve
C as a precondition to achieving B when B isn't already true, etc.
We ask how far the animal can carry the regression. Say the animals
are dog which vary in intelligence, or at least vary in the ability
to learn the tasks that humans teach dogs. We ask is there an
innate limit for dogs or can smart dogs carry it farther than dumb
dogs. %reference Kohler and Piaget and Siegler
Susan McCarthy informs me that when a performing animal is taught
a new trick, the trainer starts with the bow at the end and works
backwards. I don't know if this is related to goal regression.
\item[grammar via meaning] Many of the discussions of a child learning
its native language seem to assume that the child learns grammar
solely by observing grammatical regularities in speech and having
its grammar corrected. Consider a child raised by an English
speaking nanny whose native language is Spanish and is addicted to
Mexican soap operas. It seems to me that this happens often enough
so that observations could be made. The child would then hear a lot
of idiomatic Spanish. It would be interesting to observe whether
the child would be able to tell grammatical from ungrammatical
Spanish sentence.
My conjecture is that grammar is learned as an auxiliary to
meaning and is not separately represented in the brain.
\end{description}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{The Well-designed Logical Robot Child}
\label{sec:robot}
Ever since the 1950s, people have suggested that the easy way to
achieve artificial intelligence is to build an artificial baby and
have it learn from experience. Actual attempts to do this have always
failed, and I think this is because they were based on the Lockean
baby model.
This section concerns the design of a robot child that has some chance
of learning from experience and education. We do not mean
reprogramming, which is analogous to education by brain surgery. The
instructor, if any, should have to know the subject matter and very
no more about how the program or hardware works than parents know
about the physiology of their children.
Consider designing a logical robot child, although using logic is not
the only approach that might work. In a logical child, the innate
information takes the form of axioms in some language of mathematical
logic.
%
\footnote{There also has to be a program using the logical sentences,
and efficiency will very likely require it to use declaratively
expressed heuristics to guide its search. Very little progress has
been made in that direction, so we will ignore heuristic control
in this article.}
\cite{McC79a} and \cite{Newell:KL} both discuss using logical
sentences to represent the ``state of mind'' of a system that doesn't
use sentences directly. We don't mean that here. We are discussing a
system that uses logical sentences explicitly. If you don't like this
approach, read on anyway and then decide how your favorite approach
would handle the problems we propose to solve with logical axioms.
We will deal with just four innate structures among those mentioned in
Section \ref{sec:world}. These are {\em the relation of appearance
and reality}, {\em persistent objects}, the {\em spacial and
temporal continuity of perception} and the {\em language of
thought\/}. They are all difficult, and we can't yet go beyond
sketching the kinds of sentences that might be used by the robot
child. The design of the child robot requires many more.
\subsection{Appearance and Reality}
\label{sec:appear}
It is the essence of our approach that appearance and reality are
quite distinct and the child is designed to discover information about
reality via appearance. See \cite{McC99c}, and solve the problem it
presents of finding the reality behind and appearance. We take a
rather brute force logical attitude by making their relations
explicit.
In our formalism, both appearances of objects and physical objects
will be represented as logical objects, i.e. as the values of
variables and terms. Thus the ontology includes both appearances and
objects.
Our examples of appearances will mainly be visual appearances, because
we understand them better than auditory, tactile or olfactory
appearances. However, we would like a language that applies to
combinations of all kinds of appearances---whatever happens to be
available.
Natural language is better at describing objects than appearances.
When it has to describe appearances, it often uses objects to describe
them---as in ``a cloud shaped like a lion's head.'' This is for two
reasons. First, appearances are represented in thought by something
like pointers to the appearance itself and thus not readily
communicated. Second, appearances are fleeting and can't be fully
re-examined. Our robot's language of thought could use pointers to
pictures, e.g. gifs. These would be communicable.
Show a hungry child a picture of a hamburger and ask ``What's that?''
Answer: ``A hamburger''.
``So eat it.''
``Don't be silly. It's not a real hamburger.''
% These formulas aren't good enough.
The most obvious predicate in our logical language relates an
appearance to an object. Thus we may have a sentence
\begin{equation}
\label{appears}
Appears(appearance, object),
\end{equation}
%
in a simple context, but this simple formula requires several
elaborations.
\begin{itemize}
\item Truth of (\ref{appears}) depends on the situation. We can write
a situation calculus formula
\begin{equation}
\label{hold}
Holds(Appears(appearance, object),s),
\end{equation}
%
but we are more inclined to use the context mechanism of \cite{McC93},
although it is somewhat more complex to explain.
\item Both the appearance and the object are made up of parts, and the
correspondence of these parts often must be stated.
\item The correspondence is usually not complete. Some parts of the
appearance are artifacts or irrelevant, and some parts of the object
are not perceived.
\item It is common that the appearance changes during the lifetime of
the language of thought sentences asserting the correspondence.
\item If the correspondence is to be used to guide motor activity, we
need not merely to state that a given part of the appearance
corresponds to a leg of a certain chair but also to tell how the
orientation in appearance space of the appearance of the leg
corresponds to the orientation in physical space of the leg itself.
\end{itemize}
We need logical formulas for expressing these kinds of facts. It is
more straightforward to do when the appearance is visual than when the
appearance is tactile. How do we describe the appearance of an object
to a blind person who has not yet felt it with his hands? We share
with the blind Euclidean geometry extended to what we may call
Euclidean physics.
\subsection{Persistence of objects}
\label{sec:persist}
Maybe this part isn't so difficult now that objects are distinguished
from appearances. Objects have properties, parts and relations to one
another. They also have situation dependent locations and
orientations.
Using a {\em situation calculus\/} formalism, $Location(object,s)$
gives the location of the object $object$ in the situation $s$.
However, the orientation of an object often needs to be stated,
usually quite imprecisely.
\subsection{Conservation}
\label{sec:conservation}
According to Piaget, notions of conserved quantity come fairly late.
Piaget's classical example is asking a child whether a tall glass or a
short glass has more liquid in it just after the liquid has been
poured from one to the other. Piaget's classical result is that
children younger than about seven pick the tall glass, citing the
height.\footnote{Someone tried this on the child of a prominent AI
researcher, eliciting the answer, ``Oh, I'm not old enough to have
conservation yet''.} Siegler in his textbook \cite{Siegler98}
asserts that conservation arises more gradually with different
conservation laws being learned at different times.
Suppose something appears and disappears. There are two kinds of
mental models a person or robot can have of the phenomenon---flow
models and conserved quantity models. Flow models are more generally
applicable and apparently are psychologically more primitive. Thus
water flows from the tap onto the hands, and water flows down the
drain. This model does not require a notion of quantity of water.
The same is true of a child's early experience with money. A parent
gives you some and you buy something with it. When there is no way of
quantifying the substance, as with the water flowing from the tap, the
notion of conservation of water is of no help in understanding the
phenomenon.
Siegler considers various conserved quantities---physical objects,
numbers and liquids. Conservation of physical objects comes first.
An object that has disappeared is regarded as being somewhere, and if
the object is wanted, it is worthwhile to look for it. Conservation
of number is not apparent to first graders, and they give silly
answers to questions like $4 + ? = 7$.\footnote{Conservation of heat
wasn't apparent to 17th century Italian experimenters such as
Toricelli who used a flow model of heat. It wasn't until 1750 that
Thomas Black discovered that heat could be regarded as a quantity
that moved from one object to another by conduction.}
Let's take the designer stance. It would be good if the notion of
conservation law were innate, and experience taught which domains it
applied to. Alas, we aren't built that well.
The notion of conserved quantity is more abstract than other early
notions. The actor has to believe in there being a quantity of the
entity in question, e.g. of water. \cite{Siegler98} suggests, p. 42,
op. cit., that the child learns conservation of water via taking into
account the cross section of a glass as well as its height. My
opinion, which a suitable experiment might test, is that the abstract
notion of conserved quantity is learned, and talking about the width
of the glass is only window dressing, because not even Archimedes
could do the geometry needed to confirm the
conservation.\footnote{Archimedes assumed that the volume of the
king's crown was equal to the volume of the water it would displace,
so he didn't need to make detailed measurements on the crown.}
%[Here comes the mathematics of conservation.]
A mathematical description of a conservation law may be interesting.
Here's a situation calculus axiom saying that the amount of a
quantity $q$ is normally conserved.
Let $Amount(q,a,s)$ denote the amount of quantity $q$ in reservoir $a$
in situation $s$. We wish to say that if the occurence of the event $e$
in situation $s$ is not abnormal, then the amount of $q$ in all the
reservoirs together remains constant, i.e. $q$ is conserved.
\begin{equation}
\label{conservation}
\lnot ab(e,s) \rightarrow {\sum_{a \in A}}Amount(q,a,s) =
{\sum_{a \in A}}Amount(q,a,result(e,s)).
\end{equation}
%
The axiom (\ref{conservation}) is probably too elaborate and general
to express what real children know about conservation.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Continuity}
\label{sec:continuity}
Some philosophers and philosophically minded psychologists regard it
as odd that we perceive our experience as continuous in time even
though our nervous systems work discretely and our senses even more
discretely. The spacial continuity of our spacial perceptions, e.g.
visual, should be just as problematical. What tools should we give
our logical robot child for dealing with this? What terms should we
put in its language of thought?
I think it is to our advantage that we don't perceive the
discontinuities in our perception of continuous motion or the
discontinuous frames of a movie.
This allows us to get velocities by differentiating positions or
having the speedometer in a car do it for us. Going 50 mph usually
lasts longer than being at any particular location and is therefore
part of the situation, e.g. $Speed(Car1,s)$.
A robot child might well be designed to perceive discontinuous frames
of a TV image as continuous. Its high speed computation would allow
it to also perceive the discrete sequence of frames.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Fragments of a Language of Thought}
\label{sec:lot1}
In designing our logical robot, we can choose whether to represent and
process certain information in a serial way or a parallel
way.
%
\footnote{Some aspects of computing are going from serial to parallel
in order to achieve greater speed, but a lot of communication within
and among computers is going the other way for greater simplicity
and reliability. Multi-wire cables are being replaced by single
Ethernet or fiber optic cables. Truth and beauty are not to be
found in a single direction.}
%
In a parallel representation, a part of the information is represented
by which wire the information is on in a computer system or where in
the nervous system the information is located in a human or animal.
When all the information is in a single memory, it has to contain
labels. Our robot child will use a single memory, and therefore its
{\em language of thought} will be more explicit in representing
certain information than a human {\em language of thought} has to be.
The simplest way to represent a visual image in a computer is by a
pointer to a pixel array, e.g. in lisp as $(GIF "72385.gif")$. This
can then be the value of a variable or constant and programs can
communicate it in this form. This
has advantages and disadvantages.
\begin{itemize}
\item The information is readily displayable to a human or
transmissible to another robot.
\item Suitable programs are required if information is to be extracted
from the image.
\item If relations among parts of the image are to be expressed in
l.o.t., a suitable language for this is required. It is also
necessary to relate parts of the image to other things such as
objects. For example, we need a relation that asserts that
a certain part $foo$ of a certain picture taken from top to bottom
represents Tom's left arm. It might be written
\begin{eqnarray*}
\lefteqn{(represents} \\
&&(Top\hbox{-}to\hbox{-}bottom\ (Part\ foo\ (GIF\ "72385.gif")))\\
&&(Left\hbox{-}arm\ Tom)).
\end{eqnarray*}
\item This representation is unsuitable for mentally generated images,
whether they be invented {\em ab initio} or modified from previous
images. They will not be complete pictures.
\end{itemize}
These considerations suggest that robot l.o.t. should not represent
images primarily by pictures, although pictures might be an auxiliary
data type. Instead, curiously enough, the robot child will need
something that is closer to what we imagine human image representation
to be.
%\subsection{Learning from Experience}
%\label{sec:learning}
%One of my son's first words was ``doggie''. He used it when he saw a
%dog, when he heard barking, when he saw a picture of a dog in a book
%or a dog on TV. He also applied it to other animals. I suppose he
%would have applied it to a statue of a dog and maybe he applied it to
%various toy dogs. He also applied it to appearances of other animals.
%Likely the concept was formed well before he verbalized
%it.
%To focus the discussion we ask how our logical robot child can learn
%to recognize dogs using the tools proposed in Section
%\ref{sec:appear}.
\subsection{Consciousness}
\label{sec:consciousness}
My opinion is that self-consciousness, i.e. the ability to observe
some of ones own mental processes, is essential for full intelligence.
Whether it is essential for babies and young children is another
matter. \cite{McC96} treats the question for robots from the designer
stance, i.e. asks what self-consciouness it is useful to build into
robots.
%\subsection{Other questions about Robot Design}
%\label{sec:other-robot}
%Here are some questions.
%\begin{itemize}
%\item How shall we focus the robot child's curiosity to make good use
% of its free time?
%\end{itemize}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Remarks}
\label{sec:remarks}
The title of this essay comes from Stephen Pinker. In fact, the essay was
stimulated by his book \cite{Pinker:LI94}. While he expresses the
opinion that the mind has many built-in characteristics and favors the
idea of a language of thought, he elaborates neither idea. His
chapter on language learning is exclusively devoted to learning
grammar. I decided to see what I could do with a language of thought,
and this led to other considerations.
This isn't the best of all possible worlds.% Here's a bit of wishful
%thinking. It would be useful to have innate numerical capability---in
%senses, in thought and in output. Our senses should give us numerical
%values for angles in the visual field, frequencies and intensities of
%sound, temperature, blood sugar level, fatigue, etc. We should be
%able to do elementary physics calculations and should have $s =
%{1\over 2}gt^2$ built in. We should be able to throw a ball at a
%calculated velocity in a calculated direction. We are not supposing
%that quantities be represented by decimal numbers, just in some way
%admitting the calculations. We can give robots this capability, and
%we might be able to give it to ourselves with suitable prostheses.
%Which of the abilities that could usefully be innate are really innate
%in humans is a matter for psychological investigation. Elizabeth
%Spelke \cite{Spelke94} discusses the results of many such
%investigations.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%\section{Conclusions}
%\label{sec:conclusions}
%There will be some.
\bibliography{/u/jmc/1/biblio}
%% secretly it is /u/jmc/1/biblio.bib
%There will be more references---especially to my papers. Various
%papers are available on my Web page
%http://www-formal.stanford.edu/jmc/ and other relevant papers are
%available via the Web page of the Stanford Formal Reasoning Group
%http://www-formal.stanford.edu/.
% For leftovers see /u/ftp/jmc/child.junk=127
\vfill
{\tiny\rm\noindent /@sail.stanford.edu:/u/jmc/w95/child.tex: begun 1995 Mar 19, latexed
\jmcdate\ at \theTime}
\end{document}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Margaret Bowden, \cite{Bowden79} p34 ascribes to Piaget the idea that
a child builds analogies from, e.g. putting something into its mouth,
to putting X into Y. Piaget uses an extended meaning of imitation.
Maybe we can use this.
Science News, 1995 Sep 16, describes experiments by Andrew N.
Meltzoff, U. Washington, Seattle and commented on by Alison Gopnik,
UCB. 1995 September, Developmental Psychology. An experimental 18
month old child watches adult perform task, fail to perform task, or
just hand the toy to the child. The child performs the task if he
can, even if the adult failed, showing that the child had ascribed a
purpose to the adult. The child doesn't do task if adult didn't even
try.
Memories of appearance are usually tied to memories of structure.
A Swiss army knife is an open container with hinged blades, some
hinged at one end and some at the other. There are two short blades
(screw driver and can opener) and 3 or 4 long blades, knife, saw, fish
scaler and scissors. On one end there are recesses for a tweezers and
a toothpick. On the back there are a Philips screwdriver, a corkscrew
and three short pointy blades, one of which has a hole near the end.
%Sections needing changes before the paper is put up.
%AI experiments
%\section{Intrinsic Motivational Structure}
%\label{sec:motivation}
%From an evolutionary point of view, the basic motivation of any animal
%or plant is to pass on its genes to the next generation. However,
%this is an explicit motivation only in humans and only sometimes and
%then on the basis of considerable thought. Instead, evolution has
%produced a complex motivational structure. Here are some parts of it.
%\begin{description}
%\item [safe adventure] Children love adventure under the protection of
% an adult.
%\end{description}