There is one thing which all existing things have in common. It is the something particular to each natural body which, imparts to it an independent existence, is cause of its existence, and it determines individuality. For example, when you talk of being healthy, there must be something which is the subject of health. Movement requires something which can be moved.

Life is the function of something which is able to have life. Life, motion, or quality cannot exist apart from this something which, Aristotle calls this o usia, or primary being. The Greek word o usia is translated as substance. A better translation can be described as fundamental or independent and a primary being.

Aristotle calls air, water, and earth, o usia, primary beings because they do not belong to other beings as their attributes, but are independent beings respectively. Being can have a variety of meanings. For example, it can mean what a thing is and it can mean that a thing is this something. It is clear that primary being or o usia must refer to this something because when we say what a thing is we say it is man, we do not say that it is short or is of another quality. It is obvious that the categories cannot constitute the real being of anything, in the sense of stating what that being is. Being short is not essential to our understanding what a man is.

Similarly, to run or to bend over, doesn't attach to the being of an object, for there is always something which runs or bends over. This something exists independently of running or bending over, and does not owe its existence to its running or bending. When looking to an individual, is to look at to an individual thing, for example, to this man or woman. A man is in one way or another more than running or bending over will ever be because when speaking about him we can say something specific about him or that is unique to him. We can only talk about running by referring to something which runs. In the same aspect, we can talk about a man without having to refer to his being to another subject.

Therefore, Aristotle believes that which is primarily not in a qualified sense which is what is meant by o usia or primary being. Aristotle came up with his theory of substance at the beginning of his book Categories, where category is best defined as a most common kind of thing. The word form can itself be regarded as meaning of such a kind, so a category is a universal form. In Categories, Aristotle composed a list of the ten kinds of form. The first is substance, which is described as the kind of form which is made real by contributing to matter.

The other nine are characteristics that help us to understand what a particular substance is similar to. Aristotle explained that they are quantity, quality, relation, place, time, position, state, action, and affection. Much of Aristotle's discussion of these categories concerned the way we use such terms in language. Aristotle regards them to provide an orderly and methodical way of understanding of reality itself or the notion of substances. Aristotle further developed his realism by distinguishing between primary and secondary substances. Where primary substances are the individuals that underlie everything else, secondary substances are the characteristics which can be predicated of that individual thing.

Primary substances should be restricted to genus and secondary substances are restricted to the species to which the individual thing belongs. For instance, man is a primary substance. An animal are consequently secondary substances, which describes what kind of substance I am. Anything which is either predicated of primary substances or present in them can be regarded as a secondary substance. So, primary substances appear in the subject, while secondary substances usually appear in the predicate. Aristotle deduces that, as motion is the proper subject matter for physics, and life is the subject matter of biology, so o usia, or primary being is the subject matter of categories.

Furthermore, as each science studies its subject matter, philosophy must study all the ways something can be said to exist as an independent or primary being. In order to determine what things can be classified as o usia, the philosopher must understand the principles and reasons of fundamental beings.