A situation s is the complete state of the universe at an instant of time. We denote by Sit the set of all situations. Since the universe is too large for complete description, we shall never completely describe a situation; we shall only give facts about situations. These facts will be used to deduce further facts about that situation, about future situations and about situations that persons can bring about from that situation.
This requires that we consider not only situations that actually occur, but also hypothetical situations such as the situation that would arise if Mr. Smith sold his car to a certain person who has offered $250 for it. Since he is not going to sell the car for that price, the hypothetical situation is not completely defined; for example, it is not determined what Smith's mental state would be and therefore it is also undetermined how quickly he would return to his office, etc. Nevertheless, the representation of reality is adequate to determine some facts about this situation, enough at least to make him decide not to sell the car.
We shall further assume that the laws of motion determine, given a situation, all future situations.
In order to give partial information about situations we introduce the notion of fluent.