Introduction to the Philosophy of Saint Augustine
man is a mortal rational animal, so also every mortal rational animal is a man. The previous definition, then, was faulty because it included too much, applying to the beast as well as to man; the present one, embracing every man, and nothing besides man, is perfect.
Again, if it embraces too little, if, for example, you add "srammarian," it is likewise defective. For, even though every rational mortal animal that is a grammarian is a man, yet there are many men who are not grammarians and so are not embraced by this definition; and for this reason the statement in the first proposition is false, though its converse is true. For it is false that every man is a rational, mortal, grammarian animal; but it is true that every rational, mortal, grammarian animal is a man. When, however, neither the statement of the proposition as such nor that of its converse is true, the defect obviously is greater than when only one is false. Take these two propositions: "Man is a white animal"; again, '"Man is a four-footed animal." For, whether you say, "Every man is a white animal" or "Every man is a four-footed animal," the statements are false; and the converse statements are also false. But there is this difference, that the first applies to some men, for a great many men are white; the second does not apply to any man, for there is no man who is a quadruped. For the present, you might learn these points testing definitions, how to judge of them in the proposition and in conversion. There are many other things of this kind offered in instructions, both verbose and vague, which I shall try to teach you by an by, at the right moment.
48. Now turn your attention to that definition of ours and, applying your added experience to a study of it, correct it. Well, we had discovered that while it was a definition of sensation, it included something besides sensation, and, therefore, was not true when converted. Perhaps it is true that every sensation is an effect on the body that does not escape the awareness of the soul, just as it is true that "every man is a mortal animal." But just as it is false that "every mortal animal is a man'—for a beast is also that—so is it false that "every effect on the body that does not escape the awareness of the soul is a sensation," because our fingernails are growing now and the soul is not unaware of that, for we know it; yet we learn of it not by sensation but by inference. In the same way, therefore, as "rational" was added to the definition of man to make it exact, and the addition ruled out the beasts which had also been found included, so that we understand the definition in this form to embrace man only and all men—do you not think that our present definition requires some addition by which to exclude any