Isagoge: Chapter 13 — Of Community and Difference of Property and Difference

Property and difference, compare and contrast:

Difference also and property have it in common to be equally shared by their participants, for rational are equally rational, and risible (equally) risible (animals). Also it is common to both to be always present, and to every one, for though a biped should be mutilated, yet (the term biped) is always predicated with reference to what is naturally adapted, since also risible has the “always” from natural adaptation, but not from always laughing. Now, it is the property of difference, that it is frequently predicated of many species, as rational of divinity and man, but property (is predicated) of one species, of which it is the property. Difference moreover follows those things of which it is the difference, yet does not also reciprocate, but properties are reciprocally predicated of those of which they are the properties, in consequence of reciprocating.

The primary difference between the two is that a property of a species (in the strictest sense) is predicated only of that one species, whereas a difference can be predicated of many species. Thus, properties in this sense are reciprocal: if A can laugh then A is a man, and if A is a man then A can laugh. It seems to me, though, that property is more frequently used in the wider sense, where it is not necessarily confined to one species.

Note that properties concern the nature of the being, not the current state of the being. If I lose a leg in an accident, I am still naturally a biped; indeed, were I born with one leg, I would still naturally be a biped. A man who never laughs is still able to laugh by nature.

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