The point, then, is this: whoever already conceives the thing to be defined in and of itself will inevitably conceive what is peculiar to it and sets it apart from all else. For such a person no formal definition is necessary; it suffices to supply the name that denotes it and to indicate how the named object differs from others. A definition merely serves to alert him to the reality, just as the name does once he is familiar with what that name signifies.
By contrast, for one who has no conception of the thing, it is impossible to cite even a single attribute exclusive to it; consequently he cannot be made to know it either by a differentia or by a proprium, whether the genus and the common accident are mentioned or not. The attribute peculiar to the thing cannot be conceived without conceiving the thing itself, for it has no existence apart from it. The intellect abstracts universals only after apprehending some of their particulars; how, then, can someone who has never grasped the existing thing grasp its genus or its species?
Instead, the matter can be conveyed through exemplification and analogy, so that he apprehends the element shared between that particular attribute and its counterpart among other attributes. That shared element is neither a differentia nor a proprium, yet by combining one shared element with another one may arrive at something that singles out the thing in question, as when you say, “an intoxicating beverage.”
Accordingly, when it is said of the horse that it is a whinnying animal, of the donkey that it is a braying animal, of the camel that it is a groaning animal, of the bull that it is a lowing animal, of the sheep that it is a bleating animal, or of the gazelle that it is an animal that utters a soft bleat, these sounds are certainly peculiar to their respective species. Nevertheless, they do not define those animals for someone who was previously unfamiliar with them. Whoever does not know the horse does not know what a whinny is, and whoever does not know the donkey does not know what a bray is—that is, he does not grasp its meaning even if he hears the word. If one then attempts to define “bray” by saying, “It is the sound of the donkey,” a vicious circle results: the donkey would be known by the bray and the bray by the donkey.
The same holds for all distinctive properties: they do not acquaint a person with the thing itself unless he already knows it. It therefore becomes clear that a thing is defined only by identifying its very essence or by mentioning something akin to it; whoever has grasped the essence