On Generation and corruption   

has been affected. The altering agent, however, and the originative

source of the process are in the growing thing and in that which is

being 'altered': for the efficient cause is in these. No doubt the

food, which has come in, may sometimes expand as well as the body that

has consumed it (that is so, e.g. if, after having come in, a food

is converted into wind), but when it has undergone this change it

has passedaway: and the efficient cause is not in the food.

We have now developed the difficulties sufficiently and must

therefore try to find a solution of the problem. Our solution must

preserve intact the three characteristics of growth-that the growing

thing persists, that it grows by the accession (and diminishes by

the departure) of something, and further that every perceptible

particle of it has become either larger or smaller. We must

recognize also (a) that the growing body is not 'void' and that yet

there are not two magnitudes in the same place, and (b) that it does

not grow by the accession of something incorporeal.

Two preliminary distinctions will prepare us to grasp the cause of

growth. We must note (i) that the organic parts grow by the growth

of the tissues (for every organ is composed of these as its

constituents); and (ii) that flesh, bone, and every such part-like

every other thing which has its form immersed in matter-has a

twofold nature: for the form as well as the matter is called 'flesh'

or 'bone'.

Now, that any and every part of the tissue qua form should

grow-and grow by the accession of something-is possible, but not

that any and every part of the tissue qua matter should do so. For

we must think of the tissue after the image of flowing water that is

measured by one and the same measure: particle after particle

comes-to-be, and each successive particle is different. And it is in

this sense that the matter of the flesh grows, some flowing out and

some flowing in fresh; not in the sense that fresh matter accedes to

every particle of it. There is, however, an accession to every part of