Substance, then, is a most general genus. The body is a species of substance, and genus of the animate. The animate is a species of body, and genus of the sentient. The sentient animal is a species of the animate, and genus of the rational. The rational is a species of the animal, and genus of the mortal. The mortal is a species of the rational, and genus of man. Man is a most specific species, for he is a species of the mortal and at the same time the species of Peter and of Paul, and this is just what the holy Fathers meant by nature and form and substance.

The things which stand between the most general genus, or substance, and the most specific species, or man, ox, and so on, are subaltern genera and species. These are called essential and natural differences and qualities. They divide from those higher and are constituent of those lower; they make for the most specific species, which they constitute; and they distinguish nature from nature. Nature, moreover, is classed as most specific. Now, it has already been explained what substance and nature and form are, and what hypostasis and individual person are, and enhypostaton and anhypo-staton. It has also been explained what the difference is between substance and accidents and how substance is superior to the accidents, because in it the accidents have their existence. Division itself has also been explained, as well as how substance differs from essential differences, namely, in that the substance made specific by them constitutes a certain sort of species and becomes of such a sort. It has furthermore been explained what nature is, and what form is, and what hypostasis, and person, and individual, and what the pagan writers thought about these, and what the holy Fathers thought, they who, as disciples of the truth and of the real philosophy, were rightly teaching teachers. So come, let us now speak of the things which are proper to substance.

It is a property of the substance not to be in a subject. Rather, the substance is a subject for the existence of the accidents, but itself does not have existence in another. This is also a property of essential differences. For the being in a subject neither saves when present nor destroys when absent and hence, being entirely accidental, does not enter into the definition. Essential differences, however, are not accidents, since they do save when present and when absent they do destroy. Thus it is that they also enter into the definition.

Still another property of substance is that of being predicated univocally, that is to say, of communicating both its name and definition. Another property is that of not having any contrary. Thus, to the stone, that is to say, to the substance of the stone, there is nothing contrary. The not admitting of more or less is likewise a property, being also a property of essential differences. Thus, man is certainly no more a substance than the horse, nor is an animal either, nor is the horse more a substance than man. And there is the property of being capable of admitting contraries successively, not in itself but in its modifications. By contraries I mean those which are accidents, because the substance can by no means receive any contraries that are substantial. Thus, the rational does not admit of being irrational, but the body is heated and then by modification cooled. And a soul sometimes acquires virtue and at other times vice.

{Περὶ τοῦ τί ὑπάρχει ἴδιον τῆς οὐσίας.} Τί μὲν οὖν ἐστιν οὐσία καὶ φύσις καὶ μορφή, εἴρηται καὶ τί ὑπόστασις καὶ ἄτομον καὶ πρόσωπον ἐνυπόστατόν τε καὶ ἀνυπόστατον καὶ τίς ἡ διαφορὰ οὐσίας καὶ συμβεβηκότων, καὶ ὡς κυριωτέρα ἐστὶν ἡ οὐσία τῶν συμβεβηκότων ὡς ἐν αὐτῇ ἐχόντων τῶν συμβεβηκότων τὴν ὕπαρξιν. Εἴρηται δὲ καὶ περὶ τῆς διαιρέσεως αὐτῆς καὶ τί διαφέρει οὐσία τῶν οὐσιωδῶν διαφορῶν, ὅτι δι' αὐτῶν ἡ οὐσία εἰδοποιουμένη τοῖον εἶδος ἀποτελεῖ καὶ τοιάδε γίνεται. Εἴρηται δὲ καὶ τί φύσις καὶ τί μορφὴ καὶ ὑπόστασις καὶ πρόσωπον καὶ ἄτομον, πῶς τε οἱ ἔξω περὶ τούτων ἐδόξασαν, καὶ πῶς οἱ ἅγιοι πατέρες, οἱ τῆς ἀληθείας καὶ τῆς ὄντως φιλοσοφίας μαθηταὶ καὶ διδάσκαλοι, ὠρθοτόμησαν. Φέρε δὲ νῦν τὰ ἴδια τῆς οὐσίας εἴπωμεν. Ἴδιον τῆς οὐσίας τὸ μὴ ἐν ὑποκειμένῳ εἶναι: αὕτη γὰρ μᾶλλον ὑπόκειται τῷ συμβεβηκότι εἰς τὸ εἶναι καὶ οὐχὶ αὐτὴ ἐν ἑτέρῳ ἔχει τὴν ὕπαρξιν. Τοῦτο δὲ τὸ ἴδιον καὶ τῶν οὐσιωδῶν διαφορῶν ἐστι: τὸ γὰρ ἐν ὑποκειμένῳ οὔτε παρὸν σῴζει οὔτε ἀπὸν φθείρει: διὸ οὐδὲ ἐν τῷ ὁρισμῷ λαμβάνεται, καὶ συμβεβηκὸς γὰρ πάντως ἐστίν. Αἱ δὲ οὐσιώδεις διαφοραὶ οὔκ εἰσι συμβεβηκότα καὶ παρόντα σῴζουσι καὶ ἀπόντα φθείρουσι: διὸ καὶ ἐν τῷ ὁρισμῷ λαμβάνονται. Ἴδιον δὲ καὶ τὸ συνωνύμως κατηγορεῖσθαι ἤγουν μεταδιδόναι καὶ τοῦ ὀνόματος καὶ τοῦ ὁρισμοῦ: καὶ τὸ μὴ ἔχειν τι ἐναντίον, τῷ γὰρ λίθῳ ἤγουν αὐτῇ τῇ οὐσίᾳ τοῦ λίθου οὐδέν ἐστιν ἐναντίον: καὶ τὸ μὴ ἐπιδέχεσθαι τὸ μᾶλλον καὶ ἧττον: τοῦτο δὲ καὶ τῶν οὐσιωδῶν διαφορῶν ἐστιν ἴδιον, οὐδὲ γάρ ἐστιν ὁ ἄνθρωπος μᾶλλον τοῦ ἵππου οὐσία ἢ ζῷον οὐδὲ ὁ ἵππος μᾶλλον τοῦ ἀνθρώπου: καὶ τὸ τῶν ἐναντίων εἶναι δεκτικὸν ἀνὰ μέρος καὶ οὐ κατ' αὐτὸν ἀλλὰ κατὰ τὴν ἑαυτῆς μεταβολήν. Ἐναντίων δὲ λέγω συμβεβηκότων ἐπουσιωδῶν: τὸ γὰρ λογικὸν οὐκ ἐπιδέχεται τὸ ἄλογον, τὸ δὲ σῶμα θερμαίνεται καὶ μεταβαλλόμενον ψύχεται, καὶ ἡ ψυχὴ ποτὲ μὲν δέχεται ἀρετὴν ποτὲ δὲ κακίαν.