Cursus Philosophicus Thomisticus, Prima Pars Artis Logicae

A definition is “a discourse explaining the nature of a thing or the signification of a term.” For example, when I say, “A human being is a rational animal,” I explain the nature of a human being, which was not explained by that term, “human being.” And when I say, “A white thing is something that has whiteness,” I explain not the nature of a white thing, but the signification of the name, since it is the same as if I said, “A white thing is a word signifying that which has whiteness.” The thing defined corresponds to the definition as its object, and is convertible with it.

Hence three things must be explained: First, which conditions are required for a definition to be good; second, which conditions are required for something to be defined; third, how many kinds of definition there are and in how many ways it is made.

Concerning the first, three conditions are required for a good definition:

The FIRST is that a definition must be given through genus and specific difference. Since this condition includes not only an essential definition, in which genus and specific difference are properly found, but also an accidental and descriptive definition, in which genus and specific difference are not properly present but something standing in their place is, by “genus” we understand something common, and by “specific difference” some particular distinguishing feature. Thus the meaning is that every good definition, in order to explain some nature, ought to explain it through what it has in common with others and through what is proper to it and distinguishes it from the rest; for in this way it encompasses the whole nature of the thing.

The SECOND condition is that a definition must signify more clearly than what is defined, since it makes that evident. From this it follows that what is defined should not enter into the definition, for then it is not rendered clearer but more confused, as in the definition of a human being as a rational animal: it would not be right to add “a rational animal, which is a human being.”

The THIRD condition is that a definition be neither redundant nor incomplete. For if it contains something more or less than what is defined, it does not thereby explain its nature: it either attributes to it something it does not have, or takes away from it something it does have. Thus a definition cannot apply to anything other than what is defined; accordingly, this is the best locus for arguing from the definition to what is defined, and conversely.

Concerning the second, three conditions are likewise required for something to be defined, or to be definable:

The FIRST is that it be one per se, that is, one essence. For if a definition explains several essences, it is not one thing defined but several. Therefore, before something is defined, equivocation and the confusion arising from multiplicity must be removed; then each thing is to be defined individually. But if several things are related as one and concur to constitute a single essence, or if one is related as a thing and the other as its mode, there is no obstacle to their being comprehended in a single definition.

The SECOND condition is that what is defined be universal. For since only quiddity and nature are defined, singular things, which add individual conditions over and above the nature, cannot be defined, just as they cannot fall under a science. Singularity in common can, however, be defined, because it is then taken as a certain quiddity and nature.

The THIRD condition is that everything defined, if it is defined by a rigorous and proper definition, be some species contained under a genus, since every proper definition consists of genus and specific difference. Thus it must be given of something contained under a genus, unless perhaps it is defined through extrinsic causes or relations, which do not convey so rigorous a definition.

There are two kinds of what is defined, namely, remote and proximate. Different authors explain this in different ways, according to how they understand the notion of what is proximate and what is remote. Some say that the proximate thing defined is the very name or term signifying the thing defined, while the remote thing is the thing signified itself. For example, if the definition is that a human being is a rational animal, the term “human being” is the proximate thing defined, whereas the thing signified is remote, since things come near to us by means of terms. Hence in a definition of the meaning of a name there is only one thing defined. Others say that the proximate thing defined is the nature and quiddity itself, which falls immediately under the definition, while the remote things are those in which such a nature is found, namely, individuals. For when I define a human being, human nature is defined immediately, while Peter and its other individuals are defined remotely. Either account may be given.

Concerning the third, definition is of several kinds.

FIRST, it is divided into a definition of the meaning of a name and a definition of the thing. A definition of the meaning of a name explains the signification of the name, and thus comes closest to an etymological account; a definition of the thing explains the nature of the thing signified. For example, if I define “white” as the same as “having whiteness,” that is a definition of the meaning of the name. But if I say that it is “that which scatters vision,” that is a definition of the thing, for it explains what the thing itself is, whereas the former explains what the name signifies. In this distinction between a definition of the meaning of a name and a definition of the thing, we always proceed in the formal and per se sense. For we do not deny that through a definition of the thing we also arrive at knowledge of the proper signification, and that often the signification of a name cannot be explained without declaring the thing itself. Rather, each definition must be judged to be a definition of a name or of a thing according to what it directly and per se explains.

SECOND, a definition of the thing is divided into essential, descriptive, and causal definitions. An essential, or quidditative, definition is a discourse explaining a thing through its own parts or essential predicates, as in “A human being is a rational animal.” Since in each thing we consider physical parts, such as matter and form, and metaphysical parts, such as genus and specific difference, the quidditative definition is likewise twofold: physical, which is given through matter and form, and metaphysical, which is given through genus and specific difference. Yet in a physical definition matter is also posited in place of genus, and form in place of specific difference, so that it is true that every definition consists of genus and specific difference, or something standing in place of genus and specific difference. A descriptive definition is given through a thing’s accidents, whether proper or common, as when you say, “A human being is an animal capable of laughter,” or “a two-footed animal.” A causal definition is given through extrinsic causes. These extrinsic causes are two: the efficient and the final, as will be said in Book II of the Physics¹, because they do not constitute the nature but are outside it (for matter and form are intrinsic causes). Thus, if you define the human soul as “a form created by God for beatitude,” “by God” is the efficient cause, while “for beatitude” is the end.