Institutio Oratoria

On the other hand species will give us clear proof of genus, but is of little service for its elimination. For example, justice is always a virtue, but that which is not justice may still be a virtue, such as fortitude, constancy or self-control. Genus therefore cannot be eliminated by species unless all the species included in the genus be eliminated, as for instance in the following sentence: That which is neither rational

To these they add property and difference. Properties serve to establish definitions, differences to overthrow them. A property is that which happens to one particular object and that alone; speech and laughter for instance are properties of man. Or it may be something specially belonging to an object, but not to it alone; heating for instance is a property of fire. The same thing may also have a number of properties: light and heat are both properties of fire. Consequently, the omission of any property in a definition will impair it, but the introduction of a property, whatever it may be, will not necessarily establish a definition.

We have, however, often to consider what is a property of some given object; for example, if it should be asserted, on the ground of etymology, that the peculiar property of a tyrannicide is to kill tyrants, we should deny it: for an executioner is not ipso facto a tyrannicide, if he executes a tyrant who has been delivered to him for the purpose, nor again is he a tyrannicide who kills a tyrant unwittingly or against his will.

What is not a property will be a difference: it is, for instance, one thing to be a slave, and another to be in a state of servitude; hence the distinction raised in connexion with persons assigned to their creditors for debt: A slave, if he is manumitted becomes a freedman, but this is not the case with one who is assigned. There are also other points of difference which are dealt with elsewhere. [*](VII. iii. 26. Also III vi. 25.)