  But  McCarthy &amp; Hayes 1969  deploys a language that is much closer to first-order logic. (This formalization style is characteristic of McCarthy’s work; see  McCarthy 1979.)  Actions are treated as individuals. And certain propositions whose truth values can change over time (propositional fluents) are also treated as individuals. Where s is a situation and f is a fluent, Holds(f,s) says that f is true in s.