Book 5
10.58
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.
10.59
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.
