The axiom
states that unless something prevents it, x is on y in the situation that results from the action move(x,y).
We now list various ``things'' that may prevent this action.
Let us now suppose that a heuristic program would like to
move block A onto block C in a situation s0. The program should
conjecture from (21) that the action move(A,C) would have the
desired effect, so it must try to establish 
.
The predicate 
can be circumscribed in the conjunction of
the sentences resulting from specializing
(22), (23) and (24),
and this gives
which says that the only things that can prevent the move are the
phenomena described in
(22), (23) and (24). Whether (25) is true depends on
how good the program was in finding all the relevant statements.
Since the program wants to show that nothing prevents the move, it must
set 
, after which (25) simplifies to
We suppose that the premisses of this implication are to be obtained as follows:
1. isblock A and isblock B are explicitly asserted.
2. Suppose that the only onness assertion explicitly given
for situation s0 is
on(A,B,s0). Circumscription of 
y.on(x,y,s0) in this assertion gives
and taking 
yields
Using
as the definition of clear yields the second two desired premisses.
3. 
might be explicitly present or it might also
be conjectured by a circumscription assuming that if x were too heavy,
the facts would establish it.
Circumscription may also be convenient for asserting that when a block is moved, everything that cannot be proved to move stays where it was. In the simple blocks world, the effect of this can easily be achieved by an axiom that states that all blocks except the one that is moved stay put. However, if there are various sentences that say (for example) that one block is attached to another, circumscription may express the heuristic situation better than an axiom.
