Accident

Aristotle observed that some things are true of a thing only by happenstance. A man is an animal essentially — take that away and he ceases to be a man — but his being pale or seated belongs to him "accidentally," and he would remain himself without it. An accident is thus a property that attaches to a substance without entering into its definition, and that could have been otherwise. The distinction became foundational: later thinkers like Porphyry numbered accident among the basic ways a predicate can be said of a subject.