The Ontology in the Categories
The Categories is an early work, presumably one Aristotle wrote close to the time when he was a student in Plato's Academy. (In general, it is difficult to know when individual works in the Aristotelian corpus were written. The Categories is an exception. It is thought to be one of Aristotle's earliest works.)
In the Categories, Aristotle discusses terms. Terms are parts of language. As part of this discussion, he discusses the parts of reality to which terms correspond. In this way, the Categories contains a discussion of ontology.
In the Categories, Aristotle divides reality along two dimensions:
• object versus property
• particular versus general
Plato had emphasized the ontological division between general and particular things. Aristotle thinks that there is an equally important division that Plato overlooked. In addition to the division between particular and general, Aristotle thinks that in reality there is a division between object and property. Thus, according to Aristotle, in reality there are four kinds of things: (i) particular objects, (ii) particular properties, (iii) general objects, and (iv) general properties.
Whereas Plato included the forms among the reality of things, Aristotle thinks that the primary substances are particular objects. As primary substances, particular objects are the foundation of existence. According to Aristotle, everything else exists because the primary substances exist.
Aristotle's ontology in the Categories is more radical than it may initially appear. Consider the sentence,
'Socrates is a man.'
The term 'Socrates' in this sentence introduces a particular object, Socrates. The term 'man' also introduces an object, and this is surprising in two ways. First of all, one might think that man is not an object at all. Further, given that man is something that exists, one might think that it is a property. According to Aristotle, however, the term 'man' introduces a general object. Properties exist too, according to Aristotle. Consider the sentence,
'Socrates is pale.'
The terms in this sentence ('Socrates' and 'pale') introduce a particular object and a particular property respectively. If the sentence is true, then reality is a certain way. In this case, according to Aristotle's ontology, the particular object 'Socrates' contains the particular property that the term 'pale' signifies.
"A substance (οὐσία)--that which is called a substance most strictly, primarily, and most of all--is that which is neither said of a subject nor in a subject, e.g. this or that man (ὁ τὶς ἄνθρωπος) or this or that horse. The species in which the things primarily (πρώτως) called substances are, are called secondary (δεύτεραι) substances, as also are the genera of these species. For example, this or that man belongs in a species, man, and animal is a genus of the species; so these--both man and animal--are called secondary substances" (Categories 5.2a). "All the other things are either said of the primary substances (πρώτων οὐσιῶν) as subjects or in them as subjects. ... So if the primary substances did not exist it would be impossible for any of the other things to exist" (Categories 5.2a).
(Aristotle uses ὁ τὶς ἄνθρωπος ("this or that man") to refer to a particular man, the individual man (whoever he may be), this or that man, as opposed to ἄνθρωπος (man or human being in general).)
"It is reasonable that, after the primary substances (πρώτας οὐσίας), their species and genera should be the only other things called secondary substances (δεύτεραι οὐσίαι). For only they, of things predicated, reveal the primary substance. For if one is to say of this or that man what he is, it will be in place to give the species or the genus (though more informative to give man than animal); but to give any of the other things would be out of place--for example, to say white or runs or anything like that" (Categories 5.2b).
The Unity and Oneness of Individuals
Socrates is one thing, not a heap of things. The philosophical problem is to explain this oneness and unity.
In the Categories, the particular objects Aristotle identifies as primary substances have the unity, or oneness, characteristic of an individual because they are the atomic parts of general objects. The general object man is not divisible by any general object, but it is divisible by number. When the general object man is so divided, the many particular men are the result. These men are the indivisible and atomic parts of man. This indivisibility is what makes them be individuals. They have the unity that makes them be one object because they are indivisible.
"Every substance seems to signify a this (τόδε τι). As regards the primary substances, it is indisputably true that each of them signifies a certain this; for the thing revealed is individual (ἄτομον) and one in number. But as regards the secondary substances, though it appears from the form of the name--when one speaks of man or animal--that a secondary substance likewise signifies a certain this, this is not really true; rather, it signifies a certain qualification--for the subject is not, as the primary substance is, one, but man and animal are said of many things" (Categories 5.3b).
In the Metaphysics, Aristotle seems to change his view. He seems to think that general objects are no longer part of the ontology. This forces him to think again about the individuality of the objects he identifies as primary substances in the Categories. Since there are no general objects, individuals cannot be the atomic parts of general objects.
Just why Aristotle changes his view from the Categories to the Metaphysics is hard to know, but he may have come to think that the "this or that man" (which he cites as an example of a primary substance in the Categories) cannot be a primary substance because there is no clear account of how "this or that man" is more basic than the general objects.
This, it seems, is why Aristotle returns in the Metaphysics to the question of what is substance most of all. The result of Aristotle's inquiry remains a matter of intense scholarly debate, but Aristotle seems to argue that forms are substances and that the "this or that man" and "this or that horse" are not a primary substances because they are forms in matter.
Forms are Substances
In the Metaphysics, substances are "separable and immovable." Only divine beings meet these conditions without qualification, and hence only divine beings are substances without qualification. The forms in matter Aristotle identifies as the substances of natural bodies are separable and immovable with qualification.
"The primary philosophy treats of things which are both separable and immutable (ἡ δὲ πρώτη καὶ περὶ χωριστὰ καὶ ἀκίνητα)" (Metaphysics VI.1.1026a).
An example may help to make Aristotle's idea a little clearer.
The human soul is the form of a living human being. Since the human soul is a form, it is a substance. Since the human soul is a substance, it has the characteristics of this kind of being: it is "separable and immovable." At the same time, because the human soul is a form in matter, it is a form with qualification. Furthermore, because it is a form with qualification, it is a substance with qualification. The human soul is a substance. It is "separable and immovable" but that it is "separable and immovable" with qualification.
According to Aristotle, the human soul is the organization of material so that a living human being exists. As this organization, the soul is immune to certain changes. (This is one of the points Aristotle made against Plato. The soul makes change possible, but it does not itself change.) In this way, the human soul is "immovable." However, the soul does not exist eternally. It is fixed relative to specific behavior, but it is not fixed throughout all possible changes. In this way, the human soul is "immovable" with qualification.
Because the human soul is the organization of the material, it is in matter and is separable only in account. (Aristotle made this point against Plato. Aristotle thought it was a mistake to think that the soul could exist independently of the body.) As the organization of material, the soul cannot exist apart from the material. The soul, however, is separable in a way. Aristotle thought that the soul is separable in account. The account of the soul need not mention particular materials. To be a human soul is to exist as a rational animal.