It is best, then, to divide the genus into two, if not into three species. The species then being divided more generically, are characterized by sameness and difference. And then being divided, they are characterized by the points generically indicated.
For each of the species is either an essence; as when we say, Some substances are corporeal and some incorporeal; or how much, or what relation, or where, or when, or doing, or suffering.
One, therefore, will give the definition of whatever he possesses the knowledge of; as one can by no means be acquainted with that which he cannot embrace and define in speech. And in consequence of ignorance of the definition, the result is, that many disputes and deceptions arise. For if he that knows the thing has the knowledge of it in his mind, and can explain by words what he conceives; and if the explanation of the thought is definition; then he that knows the thing must of necessity be able also to give the definition.
Now in definitions, difference is assumed, which, in the definition, occupies the place of sign. The faculty of laughing, accordingly, being added to the definition of man, makes the whole—a rational, mortal, terrestrial, walking, laughing animal. For the things added by way of difference to the definition are the signs of the properties of things; but do not show the nature of the things themselves. Now they say that the difference is the assigning of what is peculiar; and as that which has the difference differs from all the rest, that which belongs to it alone, and is predicated conversely of the thing, must in definitions be assumed by the first genus as principal and fundamental.
Accordingly, in the larger definitions the number of the species that are discovered are in the ten Categories; and in the least, the principal points of the nearest species being taken, mark the essence and nature of the thing. But the least consists of three, the genus and two essentially necessary species. And this is done for the sake of brevity.
We say, then, Man is the laughing animal. And we must assume that which pre-eminently happens to what is defined, or its peculiar virtue, or its peculiar function, and the like.
Accordingly, while the definition is explanatory of the essence of the thing, it is incapable of accurately comprehending its nature. By means of the principal species, the definition makes an exposition of the essence, and almost has the essence in the quality.
CHAPTER VII.
ON THE CAUSES OF DOUBT OR ASSENT.
The causes productive of scepticism are two things principally. One is the changefulness and instability of the human mind, whose nature it is to generate dissent, either that of one with another, or that of people with themselves. And the second is the discrepancy which is in things; which, as to be expected, is calculated to be productive of scepticism.
For, being unable either to believe in all views, on account of their conflicting nature; or to disbelieve all, because that which says that all are unreliable is included in the number of those that are so; or to believe some and disbelieve others on account of the equipoise, we are led to scepticism.