The boundaries of a species are set by its attributes just as the boundaries of physical objects are set by their directions. Thus one may say, “The limit of this land on the southern side is such-and-such, and on the eastern side such-and-such.” By means of its name and its boundary the parcel of land is distinguished. Because one fears that something might be added to or taken away from what the name covers, a boundary is needed; it guarantees that everything intended is included and everything not intended is excluded—exactly as the name itself does. The same applies to the boundary of a species.

This clarification is achieved sometimes through verbal definitions and at other times through descriptive ones. In both cases the true purpose of a definition is simply to explain what the name denotes and to distinguish the defined thing from what is not it, not to depict the object itself. Since the benefit of a definition lies in making clear the referent of a name—and naming is a matter of linguistic convention—one must refer the issue back to the intention and language of the one who coined the term. For this reason the jurists say: some terms are defined by language, others by the Law, and others by customary usage.

Explanation and commentary of speech follow the same principle. If the goal is to clarify the speaker’s intention, this rests on knowing the limits of his words. If, however, one seeks to establish the truth and soundness of the statement, one must know the evidence that validates it. The first task portrays his words; the second affirms them.

Portraying his words is like translating the referents of names: sometimes for someone who already conceives the object but does not know that this is its name, and sometimes for someone who has not yet conceived the object at all, in which case one directs him to the referent as best one can—either by pointing to the object itself or to something analogous. Hence it is said that a definition is sometimes for the name and sometimes for the thing named.

The leading authors who practice the art of definition in the manner of the logicians acknowledge this upon close examination, as Abū Ḥāmid al-Ghazālī notes in his book Miʿyār al-ʿIlm, which he composed on logic, when he says: