There is, of course, a large literature on this ancient philosophical problem, almost none of which seems directly relevant to us. However, there is one recent theory, developed by Rescher (1964), which may be of use. Rescher's book is so clearly written that we shall not attempt a description of his theory here. The reader should be aware of Sosa's critical review (1967) which suggests some minor alterations.
The importance of this theory for us is that it suggests an alternative approach to the difficulty which we have referred to as the frame problem. In outline, this is as follows. One assumes, as a rule of procedure (or perhaps as a rule of inference), that when actions are performed, all propositional fluents which applied to the previous situation also apply to the new situation. This will often yield an inconsistent set of statements about the new situation; Rescher's theory provides a mechanism for restoring consistency in a rational way, and giving as a by-product those fluents which change in value as a result of performing the action. However, we have not investigated this in detail.