On Generation and corruption   

'passing-away' also. For if a substantial thing comes-to-be, it is

clear that there will 'be' (not actually, but potentially) a

substance, out of which its coming-to-be will proceed and into which

the thing that is passing-away will necessarily change. Then will

any predicate belonging to the remaining Categories attach actually to

this presupposed substance? In other words, will that which is only

potentially a 'this' (which only potentially is), while without the

qualification 'potentially' it is not a 'this' (i.e. is not), possess,

e.g. any determinate size or quality or position? For (i) if it

possesses none of these determinations actually, but all of them

only potentially, the result is first that a being, which is not a

determinate being, is capable of separate existence; and in addition

that coming-to-be proceeds out of nothing pre-existing-a thesis which,

more than any other, preoccupied and alarmed the earliest

philosophers. On the other hand (ii) if, although it is not a 'this

somewhat' or a substance, it is to possess some of the remaining

determinations quoted above, then (as we said)' properties will be

separable from substances.

We must therefore concentrate all our powers on the discussion of

these difficulties and on the solution of a further question-viz. What

is the cause of the perpetuity of coming-to-be? Why is there always

unqualified, as well as partial, coming-to-be? Cause' in this

connexion has two senses. It means (i) the source from which, as we

say, the process 'originates', and (ii) the matter. It is the material

cause that we have here to state. For, as to the other cause, we

have already explained (in our treatise on Motion that it involves (a)

something immovable through all time and (b) something always being

moved. And the accurate treatment of the first of these-of the

immovable 'originative source'-belongs to the province of the other,

or 'prior', philosophy: while as regards 'that which sets everything

else in motion by being itself continuously moved', we shall have to

explain later' which amongst the so-called 'specific' causes