We call necessary that without which life cannot exist, as it is a joint cause, such as breathing and nourishment are necessary for an animal; for without them it is impossible for that to exist. And these without which it is not possible for the good to be or come to be, or get rid or stripped of the bad, such as drinking the medicine is necessary so as not to be ill, and sailing to Aegina so as to get back the money.
Also, the compulsory and compulsion; this is whatever despite the inclination and choice obstructs and prevents. For the compulsory is called necessary, hence disagreeable too. But necessity, too, seems to be something that cannot be persuaded, rightly, since it is contrary to the motion produced in accordance with choice and reasoning.
Again, we say that which cannot be otherwise is necessarily so. And in this respect the necessary and the others are all called in a way necessary; for the compulsory is so called either to do or to suffer when it not possible, on account of the existing compulsion, for there being in accordance with inclination, being the necessity the cause by virtue of which it is not possible to be otherwise. The same account holds good for whatever stands as a joint cause of living and of good; for when in the one case good and in the other life and being cannot possibly be without certain things, these things are necessary and this cause is a kind of necessity.
Moreover, demonstration is of things that are necessary, because it is not possible for the conclusion to be otherwise if it has been demonstrated without exception; the reason for this being the first premisses, that is, if these from which the reasoning proceeds cannot be otherwise.
Of some things, then, the reason of being necessary is another thing, of others none, but because of them others are of necessity. So the first and mostly necessary is the simple one; for it is not possible for this to be in more than one ways, nor therefore thus at one time and otherwise at another; for in that case it would be in more than one way. Consequently, if there are some things which are eternal and unchangeable, nothing compulsory nor anything against nature is there in them.
Bibliography: Aristotle, Metaphysics (1015a.20 to 1015b.15)
Translation: George Kotsalis