ABSTRACT
There is often a practical need to describe situations whose existence is doubtful or uncertain. Planning, for example, almost by definition involves characterizing states of affairs that mayor may not arise during the course of an event. Similarly, when we give instructions, we find it useful to describe contingencies before they can occur. Reasoning and problem solving also involve depicting hypothetical circumstances whose consequences we wish to examine, and we can do this even though the circumstances are strictly impossible, as when we use reductio ad absurdum arguments to prove a theorem. In scientific contexts, experiments are based around possible outcomes of theories that are themselves initially uncertain. Finally, fiction portrays more or less elaborate situations whose relation to fact is "purely coincidental."
