primary and strict sense is the essence of things which have in themselves, as such, a source of movement; for the matter is called the nature because it is qualified to receive this, and processes of becoming and growing are called nature because they are movements proceeding from this. And nature in this sense is the source of the movement of natural objects, being present in them somehow, either potentially or actually.
Chapter 5
'The necessary' means (1) that without which, as a condition, a thing cannot live, e.g. breathing and food are necessary for an animal; for it is incapable of existing without these. — (2) The conditions without which good cannot be or come to be, or without which we cannot get rid or be freed of evil, e.g. drinking the medicine is necessary in order that we may be cured of disease, and sailing to Aegina is necessary in order that we may get our money. — (3) The compulsory and compulsion, i.e. that which impedes and hinders contrary to impulse and purpose. For the compulsory is called necessary; whence the necessary is painful, as Evenus[1] says: 'For every necessary thing is ever irksome.' And compulsion is a form of necessity, as Sophocles[2] says: 'Force makes this action a necessity.' And necessity is held to be something that cannot be persuaded — and rightly, for it is contrary to the movement
which accords with purpose and with reasoning. — (4) We say that that which cannot be otherwise is necessarily so. And from this sense of 'necessary' all the others are somehow derived; for as regards the compulsory we say that it is necessary to act or to be acted on, only when we cannot act according to impulse because of the compelling force, — which implies that necessity is that because of which the thing cannot be otherwise; and similarly as regards the conditions of life and of good, when in the one case good, in the other life and being, are not possible without certain conditions, these are necessary, and this cause is a kind of necessity. — Again, (5) demonstration is a necessary thing, because the conclusion cannot be otherwise, if there has been demonstration in the full sense; and
