Isagoge Philosophica

“Species” commonly means the outward form of things presented to sight, from which things are called beautiful, deformed, or neither. Hence Euripides’ line, “The first in beauty is worthy to rule,” to which Aristotle alludes when he says that if human beings excelled in bodily form as much as the images of the gods, everyone would judge them worthy to be served by the rest.

Among philosophers, however, “species” often names certain natural likenesses by which the senses and intellect perceive the things presented to them. We called these sensible and intelligible species above. The form that completes the substance of each natural thing is also sometimes called its species; philosophers classify this form under the genus of causes.

Their most common use of the term is for a part subjected to a genus that is itself common. In this way man is a species under animal, and laurel a species under tree. They define species as that which is subjected to a genus in answer to “What is it?” and of which the genus is predicated in answer to “What is it?” More explicitly and in a more fitting order, species is a particular that is immediately subjected to a genus in answer to “What is it?”—or, equivalently, of which the genus is predicated in this way. Man, for example, is a species subjected to animal, and animal is predicated of man in answer to “What is it?”, not “What sort of thing is it?” This subjection is immediate, without the intervention of another species.

No one should be surprised that species is used in defining genus, and genus in turn in defining species. Since genus and species are defined as universal and particular, it is useful and straightforward in defining them to indicate their mutual relation. It is therefore necessary for each to appear in the definition of the other. Otherwise it would have been enough for Aristotle to say that genus is what is predicated in answer to “What is it?” of many things that share a common nature and essence but differ among themselves.

Just as genus, by reference to the things called genera, is divided into supreme and intermediate, or subaltern, genera, so species in this sense is divided into subaltern and lowest species. Subaltern species are species that are also genera and have other species beneath them; animal, for example, is a species under living body and also a genus of man and beast. Lowest species are species of higher kinds but are not genera of lower species, and have no species beneath them; man, for example, is a species under animal but is not a genus of individual men, since human beings do not differ from one another in species, but only in number. All this has concerned species insofar as they are particulars, or parts subjected to a genus.

But since the lowest species of higher kinds are not genera of lower species, yet are predicated of them in answer to “What is it?”, another kind of universal arises. The ancients also chose to call it “species.” This is the fourth and more proper meaning of “species” for our present discussion. It is usually defined as what is predicated in answer to “What is it?” of many things differing in number. More explicitly and in a more fitting order, species is a universal that is predicated necessarily, essentially, and in answer to “What is it?” only of things that differ in number alone. In other words, it is predicated completely—or, if you prefer, immediately—of things differing only in number. In this way man is predicated only of individual human beings.

This species of universal is the most perfect of all, both with respect to the things called universals and with respect to universality or commonness itself. It is most perfect with respect to the things involved because all things called lowest species are more perfect than all the genera and differences of which they are composed, and than any of their properties and accidents. It is most perfect with respect to commonness because the unity and identity of universals with their particulars show that no other universal is as one in itself, or has as much identity with its particulars, as a lowest species has with its individuals. This is why Plato and all the other philosophers judged that one should stop at the lowest species when dividing, defining, and demonstrating: the perfection of all universal things is evident in them. Individuals are indeed more perfect and more beings than lowest species, as Aristotle teaches, but they are rightly left aside, partly because their great multitude cannot be reckoned by a determinate number so as to fall within our science, and partly because all things under the same lowest species have equal perfection of nature. Thus someone who has learned about even one of them need not investigate the others. Species are contrary to individuals in both these respects. With a few exceptions—which arise either from unusual natural unions of animals, from the arts of grafting and of applying active agents to passive subjects as these are gradually discovered, or from the chance combination of several natural causes, knowledge of which in time enriches the arts and sciences—all other species are fixed and determinate in number, though most are unknown to us. The same is true of subaltern genera.

Thus, under the supreme genus substance, we count corporeal and incorporeal substance; under corporeal substance, corruptible and incorruptible; under corruptible, living and nonliving; under living, sensitive and non-sentient, which we call animal and plant; under animal, man and brute animal; but under man there is no determinate number of individuals. In the same way we can descend through the species we pass over here until we reach the lowest species in each order. It is not always necessary to divide a genus in two. Although corruptible substance is divided into simple and mixed, simple substance is divided among the four species of the elements, and plants into three genera of plants—trees, shrubs, and herbs. The same occurs in many other genera, which are divided into more than two species but still into a finite number. Thus, whether one descends from the supreme genera or proceeds across the divisions, one will never encounter an infinite number of true and genuine species. We set aside those that could exist in nature only by God’s absolute, rather than ordinary, power, since we are not concerned with their number.

Everyone agrees that no species can be entirely equal to another in perfection. For, as Aristotle teaches, species of things are like species of numbers: just as no species of number is equal to another in its multitude of units, so no species of things is equal to another in perfection. Aristotle’s statement applies properly to species of which some are higher and others lower; in its own way it applies to the others as well, since all, as he also teaches, hold among themselves an order of prior and posterior according to their own natures. Thus colors nearer to the first and most perfect color are held to be more perfect, and animals nearer to the first and most perfect animal must likewise be more perfect.

Moreover, just as genera are naturally prior to their species in the order of generation, so species are prior to their genera in the order of perfection. It follows that one may proceed from species to genera by a consequence of either being or existence, but neither consequence runs in reverse, from genera to their species.

Likewise, just as species are better known to us in the order of discovery, genera are better known in the order of exact understanding. In investigating common natures, we customarily proceed from lower kinds, which are closer to the senses, to higher ones; but in dividing, defining, and demonstrating them, we descend from higher to lower.

Finally, by ascending from species to genera and from individuals to species, we gather one from many; by descending from genera to species and from species to individuals, we make many from one. Through participation in a species, many men are one man; but what is one and common becomes many through the addition of particular conditions. The same can be said of animal in relation to man and its other species. This is enough about genus and species.