spect, so that it is impossible to know the one without the other, wherefore in the definition of the one, the other must of necessity be comprehended. All such things, then, it is necessary to know, and to employ them as may appear useful.
Another (place) is, if in the definition the thing defined is used, but this is latent when a person does not employ the very name of the thing defined, as if he should define the sun to be a star apparent by day, for using day, he uses sun. In order to detect such, we must take the definition instead of the name, as the day is the motion of the sun above the earth; for it is clear that he who speaks of the motion of the sun above the earth, mentions the sun, so that he who uses day, uses sun.
Again, if what is in an opposite division is defined by what is in an opposite one, as the odd to be what is greater than the even, by unity: for things oppositely divided from the same genus are naturally simultaneous, but the odd and even, are divided oppositely, since both are differences of number.
Similarly also if the superior is defined through the inferior, as that the even number is what may be divided into two parts, or that good is the habit of virtue; for the expression, "into two parts," is assumed from two, which is an even number; virtue also is a certain good, so that these are under those. Besides, it is necessary that whoever uses the inferior should use also (the thing defined) itself; for both he who uses virtue uses good, since virtue is a certain good, and likewise also he who uses "in two parts" uses the even, because a division into two parts, signifies to be divided into two, but two is an even number.
Chapter 5
Universally then, one place is, that a definition is not framed through things prior and more known, but the particulars of it are such as have been mentioned. The second place is, if when a thing is in genus it is not placed in genus, but in all such, an error occurs in the definition of which, what a thing is, is not previously declared; for instance, the de-
