In the original version of the situation calculus, fluents are not reified. In other words, conditions that can change are represented by predicates and not by functions. Actually, McCarthy and Hayes defined a fluent as a function that depends on the situation, but they then proceeded always using predicates to represent fluents. For example, the fact that it is raining at place formula_73 in the situation formula_27 is represented by the literal formula_75. In the 1986 version of the situation calculus by McCarthy, functional fluents are used. For example, the position of an object formula_73 in the situation formula_27 is represented by the value of formula_78, where formula_79 is a function. Statements about such functions can be given using equality: formula_80 means that the location of the object formula_73 is the same in the two situations formula_27 and formula_83.