On a rational view of definition, a definition organizes and condenses — and thus helps one to retain — a wealth of knowledge about the characteristics of a concept's units. On the nominalist view, it is precisely this knowledge that is discarded when one defines a concept: as soon as a defining characteristic is chosen, all the other characteristics of the units are banished from the concept, which shrivels to a mere definition. For instance, as long as a child's concept of "man" is retained ostensively, the child knows that man has a head, two eyes, two arms, etc.; on the nominalist view as soon as the child defines "man," he discards all this knowledge; thereafter, "man" means to him only: "a thing with rationality and animality." [IOTE, 104]
Note the complete absence of empirical examples to support Peikoff's contentions: which nominalists, after all, believe the people discard knowledge after defining words? Is it too much to ask for names, followed by documented evidence? This absence of evidence is not only intentional, but necessary: there can be no examples because it's unlikely any nominalist ever held the position attributed them by Peikoff in this passage. What Peikoff and other Objectivists still can't seem to grasp is that definitions define words, not concepts. The definition explains how a particular word is to be used. It gives the meaning of the word in different terms. Neither Rand nor Peikoff ever bothered to provide evidence for the assertion that definitions organize and condense knowledge about the characteristics of a concept's units. Since definitions only express the same meaning in different words, they add no knowledge about external matters of fact (other than knowledge about word usage).