Our insight into the complementary functions of concept formation and measurement provides the clue to the solution of the problem of universals (pp. 1.3:12-3). Ayn Rand has defined a "concept" as "a mental integration of two or more units possessing the same distinguishing characteristic(s), with their particular measurements omitted" (Introduction to Objectivist Epistemology, p. 15; more information). Each distinguishing characteristic, in other words, must be present in some degree, but may be present in any degree. Its specific measurement may vary among the units subsumed within the concept.

Imagine, for instance, that the smiley population had included various sizes of torsos for both the roly-polies and the boxies, such that no two specimens had the same torso size. Then from a less sophisticated point of view, it might now seem that no specific feature is the same among all the boxies, yet absent from the other smileys. After all, no two boxies have identical torsos, just as no two human beings have an identical faculty of "rationality."