that conception except after knowing that the definition is sound, for the definition is the evidence for that conception, the means of attaining it, and what reveals it. It is therefore impossible to know the thing defined before knowing that its definition is sound. Yet knowledge of the definition’s soundness can be attained only after knowing what is defined. For a definition is a report about a subject [of the report] (1), namely, what is defined. It is impossible to know that a report is sound and true before conceiving its subject, without uncritically accepting the report and taking the reporter at his word concerning something of which both the reporter and the recipient have knowledge. This is not a case of reporting matters beyond one’s observation.

Fourth:

They define a thing through attributes that they call essential and accidental. They call these the parts of the definition, the parts of the essence, its constitutive elements, those included within it, and similar expressions. If the listener does not know that the thing defined possesses those attributes, he cannot conceive it. If he knows that it possesses them, he has already conceived it without the definition. Thus, in either case, he has not conceived it through the definition. This is clear.

For when it is said, “A human being is a rational animal,” someone who does not know that this is a human being needs to know this relationship. If he has not conceived what “rational animal” denotes, he needs two things: to conceive it and to know the relationship just mentioned. If he knows this, he has already conceived a human being without the definition.

Admittedly, a definition may bring the conception of what is defined to mind, just as a name does. The mind may be inattentive to something; upon hearing its name alone, a person turns his mind towards the thing indicated by the name or definition and conceives it. Thus, the benefit of a definition is of the same kind as that of a name, and definitions of species through their attributes are like boundaries