Isagoge Philosophica

Taken broadly, “genus” is used in many ways. We often misuse the name of a genus for a species or something like a species, as when we say there are many genera of languages or colors, and generally when we bring something more common down to less common things—even if those less common things have nothing common beneath them but are themselves singulars. Properly speaking, however, “genus” seems to have about four meanings.

First, the continuous and uninterrupted generation of things of the same species is called a genus, as it were a common begetting, without any regard to a first progenitor. The term is also applied in the same sense to the whole multitude of begetters and begotten in an unbroken succession. This is how we use it when we say, “As long as the human race shall endure”—that is, as long as their generation continues, or the succession of begetters and begotten remains unbroken. In this sense they would not be called one genus if the common flood of the whole world had killed all the people then alive and others had been brought forth in some way other than generation from a woman.

“Genus” is also used for the very origin of an uninterrupted line of generation from a single progenitor.

In this sense we say that all men ever born of a woman trace their lineage to Adam, and all those born after the universal flood to Noah; we also speak of Greeks as descended from Graecus and Ionians from Ion. We readily extend this usage to any family, in which we find two relations: one of natural dependence on the first parent and head of the whole group, the other of kinship among all its members. By these two relations we distinguish each such lineage from others of the same kind. Related to this meaning is the use of “genus” for origin from a homeland or nation, since a nation may be called a kind of broader homeland. Aeneas, for example, seeks Italy as his homeland and claims descent from Jupiter; in the same verse the poet recalls both kinds of lineage, that from a homeland and that from a first progenitor. In this way we call Plato Athenian by nation and Aristotle a Stagirite; or, more precisely, Plato an Achaean and Aristotle a Macedonian by nation, but the Stagirite and the Athenian by homeland. For although a homeland has greater force in the generation of each person, a nation contributes more broadly to it.

A third use, proper to philosophers, takes genus as a subject in which various differences are found. Thus surface is called the genus of plane figures and solid the genus of solid figures. Different sciences and arts concern different genera of subjects, and each demonstrates the proper attributes of its own subjects; as Aristotle says, one science may not pass into the genus of another.

The fourth and principal meaning, most familiar to philosophers, calls genus that to which species are subjected. Some features of the two preceding meanings resemble this one. Genus in this sense is a kind of principle and source of its species, into which it is, as it were, propagated; in the composition of its species it plays, as it were, the role of matter and of a subject for opposed differences. In describing it, Aristotle says that a genus is what is predicated of many things differing in species in answer to “What is it?” To express all that this description implies, and to arrange its terms so that things alien to the account of genus are excluded in the proper order, genus should be said to be a certain universal predicated necessarily and essentially, in answer to “What is it?”, of things differing in species—that is, as a complete expression of their common nature. Once genus is explained in this way, individuals are first excluded, since they are furthest from the account of genus; next accidents, which are the lowest class of universals; then properties, which belong necessarily; then differences, which already pertain to essence; and finally species, which alone among the universals completely contain the common nature of singular things.

These are not, however, several differencee by which a genus is constituted in the order of generation, such as those used to define man when we say that he is a corporeal, corruptible, living, sensitive, rational substance. Rather, they indicate a single difference, whatever it may be, with the full force conveyed by those several terms taken separately. It follows that we should not look for intermediate species between the universal and its five familiar species, into which those five might be divided—any more than we look for intermediate species of colors between color and the lowest colors—even though some of the latter agree more closely with one another than with others. This account differs from the deeper definition of genus only in using the word “predicated” in place of the verb “to be,” which signifies the basis of predication. It can therefore serve, in its own way, as an essential definition: Genus is a universal that is necessarily and essentially, or quidditatively, in many things differing in species. These two points should be recalled when considering the similar definitions of the other universals.

Genus is divided, by reference to the things called genera, into supreme and subaltern genera. A supreme genus has no genus above it and, though it is a genus, is not also a species; substance, quantity, and quality are examples. A subaltern or intermediate genus has another genus above it and, while a genus, is also a species; corporeal substance and continuous quantity are examples.

If we are speaking of real and complete things, there are ten supreme genera: substance, quantity, quality, relation, action, passion, where, when, position, and possession. They are regarded as the first principles and, as it were, sources of the categories. Being, which might seem to be the common genus of all things, does not have the account of a genus. For, as Aristotle teaches, although it is not altogether equivocal, it is not univocal either; it is something intermediate, which is called analogical. We will treat more conveniently, in the explanation of species, the multitude of subaltern or intermediate genera, since they are both genera and species.