De principiis naturae

It should also be known that, in speaking of the intrinsic principles, namely matter and form, there is sameness and difference of principles according to the sameness and difference of the things principled. For some things are the same in number, as Socrates and "this man" with Socrates pointed out; some are diverse in number and the same in species, as Socrates and Plato, who though they agree in the human species yet differ in number. Some, however, differ in species but are the same in genus, as man and ass agree in the genus animal; while some are diverse in genus but are the same only according to analogy, as substance and quantity, which do not agree in any genus but agree only according to analogy: for they agree only in this, that each is a being, and being is not a genus, since it is predicated not univocally but analogically.

For the understanding of this it should be known that a thing is predicated of many in three ways: univocally, equivocally, and analogically. That is predicated univocally which is predicated according to the same name and according to the same account, that is, definition, as animal is predicated of man and of ass: for each is called animal, and each is an animated sensible substance, which is the definition of animal. That is predicated equivocally which is predicated of some things according to the same name and according to a diverse account, as dog is said of the barking dog and of the heavenly Dog, which agree only in the name and not in the definition or signification; for what is signified by the name is the definition, as is said in Book IV of the Metaphysics. That is said to be predicated analogically which is predicated of many whose accounts are diverse, but are attributed to one same thing, as healthy is said of the body of an animal and of urine and of a potion, but it does not signify altogether the same in all: for it is said of urine as of a sign of health, of the body as of its subject, of the potion as of its cause. Yet all these accounts are attributed to one end, namely health.

For sometimes things that agree according to analogy, that is, in proportion or comparison or agreement, are attributed to one end, as is clear in the example just given; sometimes to one agent, as doctor is said both of him who acts through the art and of him who acts without art, as an old woman, and also of instruments, but by attribution to the one agent which is medicine; sometimes, however, by attribution to one subject, as being is said of substance, of quality and quantity and the other predicaments: for the account by which substance is a being is not altogether the same as that by which quantity and the others are, but all are so called from the fact that they are attributed to substance, which is the subject of the others. And therefore being is said priorly of substance and posteriorly of the others; and therefore being is not the genus of substance and quantity, since no genus is predicated priorly and posteriorly of its species, but it is predicated analogically. And this is what we said, that substance and quantity differ in genus but are the same by analogy.

Of those things, therefore, which are the same in number, the form and matter are the same in number, as of Tully and Cicero; of those, however, which are the same in species but diverse in number, the matter and form too are not the same in number but in species, as of Socrates and Plato. And likewise of those which are the same in genus, the principles too are the same in genus, as the soul and body of an ass and a horse differ in species but are the same in genus. And likewise of those which agree only according to analogy, the principles are the same only according to analogy or proportion. For matter and form and privation, or potency and act, are the principles of substance and of the other genera; yet the matter of substance and of quantity, and likewise the form and privation, differ in genus, but agree only according to proportion in this, that as the matter of substance stands to substance in the character of matter, so the matter of quantity stands to quantity. Yet as substance is the cause of the rest, so the principles of substance are the principles of all the others.