   In the terminology introduced in POM, constituents of a proposition occur either “as term” or “as concept”.  An entity occurs “as term” when it can be replaced by any other entity and the result would still be a proposition, and when it is one of the subjects of the proposition, i.e., something the proposition is “about”. An entity occurs as concept when it occurs predicatively, i.e., only as part of the assertion made about the things occurring as term. In the proposition Socrates is human, the person Socrates (the man himself) occurs as term, but humanity occurs as concept. In the proposition Callisto orbits Jupiter, Callisto (the moon itself) and Jupiter (the planet) occur as term, and the relation of orbiting occurs as concept. Russell used the word “concept” for all those entities capable of occurring as concept—chiefly relations and other universals—and the word “thing” for those entities such as Socrates, Callisto and Jupiter, that can only occur as term. While Russell thought that only certain entities were capable of occurring as concept, at the time, he believed that every entity was capable of occurring as term in a proposition. In the proposition Wisdom is a virtue, the concept wisdom occurs as term. His argument that this held generally was that if there were some entity, E, that could not occur as term, there would have to be a fact, i.e., a true proposition, to this effect.  However, in the proposition E cannot occur as term in a proposition, E occurs as term (POM, 44–45).