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 ousia, or primary being.
The Greek word ousia is translated as substance. A better translation can be described as fundamental or independent and a primary being. Aristotle calls air, water, and earth, ousia, 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 ousia 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 ...