Of course, human beings are distinguished from other animals by many different characteristics. In order for our definitions effectively to serve their purpose (helping us to distinguish a concept's units mentally), they should specify those distinguishing characteristics upon which the largest number of other such characteristics are known to depend. Such characteristics are said to be epistemologically essential. Although the essential characteristics are always determined by this objective standard, that standard must be applied to a given context of knowledge. For example, a child's definition of human, formulated within a narrower context of information, might possibly be different—and yet be objectively correct within that context. Thus a concept's definition, unlike its meaning, may vary according to one's context of knowledge.

In our adult definition of human, "rational" implies merely that humans possess a faculty of reason (i. e., conceptual consciousness) and includes no suppositions about specific measurements of that faculty. The definition even includes humans in whom the rational faculty may be largely atrophied from disuse, who are often described as thinking and acting "irrationally."