Part II, Chapter 16: Of the Definition which is termed the Definition of Things
WE have spoken at considerable length, in the First Part, of the definition of names, and we have shown that we must not confound it with the definition of things, since the definitions of names are arbitrary, whereas the definitions of things do not depend on us, but on what is involved in the true idea of the thing, and are not to be taken as principles, but considered as propositions, which need after to be established by reason, and which may be disputed. It is, then, of this last kind of definition alone, that we speak here.
Of this there are two kinds,—the one more exact, which retains the name of definition; the other less so, which is termed description.
The more exact, is that which explains the nature of a thing by its essential attributes, of which those which are common are called genus, and those which are special, difference. Thus we define man, a rational animal; mind, a substance which thinks; body, a substance extended; God, a perfect being. It is necessary, too, as far as possible, that which is placed as genus in the definition, be the proximate genus of the thing defined, and not simply the remote.
Sometimes, also, we define by integral parts, as when we say, that man is a thing compounded of mind and body. But even then there is something which holds the place of genus—the term thing compounded, and the rest takes the place of difference.
The definition less exact, which is termed description, is that which gives some knowledge of a thing by the accidents which are peculiar to it, and which determine it sufficiently to enable us to discriminate it from others. It is in this way that we describe herbs, fruits, animals, by their figure, size, colour, and other such accidents. The descriptions of poets and orators are of this nature. There are also some definitions or descriptions of things by their causes, matter, form, end, &c.; as if we define a clock, an iron machine, composed of different wheels, whose regular movement is intended to mark the hours.
There are three things necessary to a good definition,—that it be universal, that it be appropriate, and that it be clear.
1. It is necessary that a definition be universal, that is to say, that it comprehend the whole thing defined. Hence the common definition of time, that it is the measure of motion, is probably bad, since it is very likely that time measures rest as well as motion. For we say that a thing has been so long at rest, as well as that it has been moving for so long a time; so that it is clear that time is nothing more than the continuance of a creature in some state, whatever that state may be.
2. It is necessary that a definition be special, that is to say, that it belong exclusively to the thing defined. Hence the common definition of the elements, as simple corruptible bodies, seems bad; for the celestial bodies, being not less simple than the elements, by the confession of these philosophers themselves, we have no reason to suppose that the heavens are subject to alterations like those which take place on earth, without speaking of comets, which we now know are not formed from the exhalations of the earth, as Aristotle imagined. There have been discovered spots on the sun, which have formed and dispersed there in the same way as our clouds, although they are of much greater magnitude.
3d, A definition must be clear, that is to say, it must serve to give us a clearer and more distinct idea of the thing which we define, and that it enable us, as far as possible, to comprehend its nature, so that it may help us to give an account of its principal properties, which is what ought principally to be considered in definitions, and what is neglected in a great number of Aristotle’s definitions. For who is there that ever comprehended the nature of motion better through this definition: Actus entis in potentia quatenus in potentia,—the act of a being in power as far as it is in power? Is not the idea which nature gives us of it a hundred times more clear than this? and who is there that has ever learned from it any of the properties of motion?
The four celebrated definitions of these first four qualities, the dry, the moist, the hot, and cold, are no better. The dry, says he, is that which is easily retained within its own limits, and with difficulty in those of another body,—Quod suo termino facile continetur, difficulter alieno.
And the moist, on the contrary, is that which is easily retained in the boundaries of another body, and with difficulty in its own,—Quod suo termino difficulter continetur, facile alieno).
But in the first place, these two definitions belong more to hard and liquid bodies, than to dry and humid bodies; for we say that one air is dry, and that another air is humid, though it may be always retained within the bounds of another body, because it is always fluid. And further, we do not see how Aristotle could say that fire, that is, flame, is dry, according to this definition, since it easily accommodates itself to the limits of another body; whence, also, Virgil calls fire liquid, et liquidi simul ignis; and it is vain subtlety to say, with Campanella, that fire, when confined, aut rumpit aut rumpitur; for this is not because of its pretended dryness, but because its own smoke stifles it if it has no air. Hence it is easily confined within the limits of another body, provided there be any opening through which it may discharge that which it constantly exhales.
Hot, he defines, that which collects like bodies, and separates unlike,—Quod congregat homogenea, et disgregat heterogenea, And cold, that which collects unlike bodies, and separates like,—Quod congregat heterogenea, et disgregat homogenea. This sometimes belongs to cold and hot, but not always; but it does not at all enable us any better to understand the true cause which leads us to call one body hot, and another cold. So that the chancellor Bacon had reason to say that these definitions were like to that which one might make of a man, in defining him to be an animal that made shoes, or cultivated vines. The same philosopher defines nature, Principium motus et quietis in eo in quo est,—The principle of motion and of rest in that in which it is; which is founded on a fancy that he had, that natural bodies differed from artificial bodies in this, that natural bodies had within them the principle of their movement, and that artificial bodies had it only from without; whereas it is clear and certain that no body can impart motion to itself, because matter, being of itself indifferent to motion or rest, cannot be determined to one or the other except by a foreign cause. And since we cannot go on to infinity, it must necessarily be God who has impressed motion on matter, and who preserves it in it still.
The celebrated definition of the soul appears still more defective: Actus primus corporis naturalis organici, potentia vitam habentis,—The first act of a natural organised body having life in potentia. We do not know what he intends to define. For, 1st, if it is the soul, so far as it is common to men and beasts, he is defining a chimera, there being nothing common to these two things. 2d, He is explaining an obscure term by four or five more obscure. And to refer only to the word life, the idea which we have of life is not less obscure than that which we have of the soul, these two terms being equally ambiguous and equivocal.
These are some of the rules of division and definition. But although there is nothing more important in the sciences than to divide and define well, it is unnecessary to say more about it here, as it depends much more on a knowledge of the matter treated of than on the rules of logic.