I learned two new terms this week, courtesy of Peter Kreeft’s Socratic Logic, that are immediately applicable to De Ente et Essentia. The terms are extension and comprehension, two properties that terms have. (He also cited four pairs of synonyms; apparently, the concepts are important but the names for them are little agreed upon.)
Simply put, the comprehension of a term is its meaning; and the extension of a term is the set of things to which it refers. The comprehension of the term man, for example, is rational animal; the extension of the term man, is all men and women.
The interesting thing about comprehension and extension is that they are inversely related. If you increase the comprehension of a term, you decrease its extension, and vice versa. Thus, the term rational animal has greater comprehension than the term animal alone, it is more determinate, but at the same time it has a lesser extension, it refers to fewer beings.
And this, it so happens, is the distinction that Thomas has been making in Chapter 2 of De Ente et Essentia. In terms of extension, the genus animal contains everything that is in the species man; but in terms of comprehension the term animal means only that which all animals have in common, and is thus only a part of the species man.
Some books just seem to come along at the right time.