Yet if we mentally isolate the attribute "square torso," omitting specific measurements of torso size, it becomes clear that this abstract attribute applies uniquely to the boxy subclass, and not to the roly-poly subclass. It is also clear that such a mental process is not arbitrary, but derives from real differences among the smileys. Thus we do have an objective basis for the concept "boxy." In the same manner, we can form a valid concept of "human," based on the rational faculty (i. e., reason) as a distinguishing characteristic—even though that faculty may (like smiley torsos) differ considerably from human to human. Such a concept is truly objective—that is, it is based purely on recognition of facts of reality.

Several fundamental thinking errors arise from confusing concept formation with measurement. These errors fall in two general categories:

attempts to measure differences of kind or to include them on a common scale of measurement; and
attempts to base conceptual distinctions on differences of degree, as if they were differences in kind.

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