As observed earlier (p. 1.3:3), the meaning of a proposition depends on the meanings of its constituent concepts. Similarly, if a concept is truly a mental integration of the units it subsumes, then its meaning lies in all of the existents from which it is abstracted. In other words, the meaning of a concept includes all of the characteristics—known and as yet unknown—of its constituent units. This point becomes clearer when we consider how a concept such as "human" is used in propositions. Obviously, the truth or falsity of some assertions about humans cannot be established merely by examining our definition of the term "human," or even by considering our present limited knowledge of humans.

A valid concept, as we have seen, is not formed arbitrarily but derives from objective properties of its constituents—properties in which they are similar in kind to each other but different from other units. Pseudo-concepts—that is, groupings not based on such objective properties—not only impede understanding but also lead to endless and fruitless arguments, especially with regard to the complex issues that we will be examining in future sections. We will therefore take great pains in this course to ensure that our use of concepts such as the "good," "justice," "rights," and "freedom" is based not on subjective opinion, but on fundamental differences of kind among existents, differences that must be properly identified if we are to understand reality.