As to whether we are to suppose that the _substances_ as well as their _qualities_ undergo this intimate mingling, as Zeno of Citium afterwards declared, I do not think it necessary to go further into this question in the present treatise;[17] for immediate purposes we only need to recognize the _complete alteration of substance_. In this way, nobody will suppose that bread represents a kind of meeting-place[18] for bone, flesh, nerve, and all the other parts, and that each of these subsequently becomes separated in the body and goes to join its own kind;[19] before any separation takes place, the whole of the bread obviously becomes blood; (at any rate, if a man takes no other food for a prolonged period, he will have blood enclosed in his veins all the same).[20] And clearly this disproves the view of those who consider the elements[21] unchangeable, as also, for that matter, does the oil which is entirely used up in the flame of the lamp, or the faggots which, in a somewhat longer time, turn into fire. I said, however, that I was not going to enter into an argument with these people, and it was only because the example was drawn from the subject-matter of medicine, and because I need it for the present treatise, that I have mentioned it. We shall then, as I said, renounce our controversy with them, since those who wish may get a good grasp of the views of the ancients from our own personal investigations into these matters. The discussion which follows we shall devote entirely, as we originally proposed, to an enquiry into the number and character of the _faculties_ of Nature, and what is the effect which each naturally produces. Now, of course, I mean by an effect[22] that which has already come into existence and has been completed by the _activity_[23] of these faculties--for example, blood, flesh, or nerve. And _activity_ is the name I give to the active change or _motion_, and the _cause_ of this I call a _faculty_. Thus, when food turns into blood, the motion of the food is passive, and that of the vein active. Similarly, when the limbs have their position altered, it is the muscle which produces, and the bones which undergo the motion. In these cases I call the motion of the vein and of the muscle an _activity_, and that of the food and the bones a _symptom_ or _affection_,[24] since the first group undergoes _alteration_ and the second group is merely _transported_. One might, therefore, also speak of the _activity_ as an _effect_ of Nature[25]--for example, digestion, absorption,[26] blood-production; one could not, however, in every case call the effect an activity; thus flesh is an effect of Nature, but it is, of course, not an activity. It is, therefore, clear that one of these terms is used in two senses, but not the other. II Kai zêtêsomen kata tonde ton logon, hypo tinôn gignetai dynameôn auta dê tauta kai ei dê ti allo physeôs ergon estin.