Tom Costello [] has proposed generalizing the notion of persistence. Instead of a fluent persisting, he proposes that a fluxion (a further borrowing from Newton) persists. A fluxion is a function of time. The following is an adaptation of his idea.
Consider a lemming walking. Consider the following formulas:
where v is the velocity of the lemming, and we have the definition
walking is thus treated as an ordinary persistent fluent in
(10). Its effect on the position of the lemming is given by
the auxiliary formula (11).
A persistent fluent continues to hold until something stops it, and then it ceases to hold.
For completeness, we should also include
but there isn't much that lasts forever.
