The second type of error—attempting conceptual distinctions based on differences of degree—was previously encountered in our discussion of smileys. Smiley leg-length, we found, was not in itself a valid basis for subdivision of the "smiley" concept. Later, however, an additional environmental factor was discovered—one that might cause a true difference in kind between smileys with leg-lengths above and below a certain threshold (pp. 1.3:25-6). As was pointed out, differences in degree often give rise to such differences in kind when we begin to categorize living beings (and other functional entities), because the life-function of organisms often depends on specific degrees of certain factors. For instance, human life and health require a certain range of temperatures, certain pH levels, certain amounts of various nutrients, and so forth.

From this commonplace observation, unfortunately, some people attempt to generalize by elevating "moderation" into a universal guiding principle. Thus we are admonished to temperance in all things, to follow the middle path and never go to extremes. In political rhetoric, it is popularly accepted as an axiom that "extremism" is always wrong and the "middle road" best—regardless of the principle involved. In a three-way debate, for instance, the "middle" position is typically characterized as "the cool voice of reason" and commands the sympathy of much of the audience, no matter how logically muddled that position may be.