John, responding to the relation P+Q, engages in Act X.
But a world that contains X is a world in which the relation P+Q does not hold.
This is, I think though to test it would require an immense amount of historical information, mostly a feature of capitalist social orders. As follows:
There is a great demand for attorneys.
Mary Smith, recognizing this demand, goes to law school. (So do 10s of thousands of others.)
Because Mary went to law school there is no longer a demand for attorneys.
Or (pre-capitalist)
Laius & Jocasta order the death of the child who is fated to kill Laius and marry Jocasta.
Therefore Oedipus (not knowing his parentage) kills Laius and marries Jocasta.
If only they had not attempted to avoid fate they would have avoided fate.
If only Mary had not gone to law school she could have become an attorney.!!!!! As this operates under developed capitalism it seems that even after the fact one cannot see how the result could have been prevented -- i.e., under capitalism the gap between motive and act becomes absolute, though (as with Odipus) it was a special case rather than a universal feature in pre-capitalist social orders.
On the cusp between pre-capitalism and capitalism, Shakespeare's Farmer who killed himself in expectation of plenty.
An act changes the conditions which motivated it and in so doing creats a world in which that act is contra-indicated.
Ian loves to play with logic. Perhaps he can improve on this.
Carrol