Thus we shall enquire, in the course of this treatise, from what faculties these effects themselves, as well as any other effects of nature which there may be, take their origin.
First, however, we must distinguish and explain clearly the various terms which we are going to use in this treatise, and to what things we apply them; and this will prove to be not merely an explanation of terms but at the same time a demonstration of the effects of nature.
When, therefore, such and such a body undergoes no change from its existing state, we say that it is at rest; but, notwithstanding, if it departs from this in any respect we then say that in this respect it undergoes motion.
Motion (kinesis) is Aristotle's general term for what we would rather call change. It includes various kinds of change, as well as movement proper. cf.Introduction, p. xxix.|
Accordingly, when it departs in various ways from its preexisting state, it will be said to undergo various kinds of motion. Thus, if that which is white becomes black, or what is black becomes white, it undergoes motion in respect to colour
; or if what was previously sweet now becomes bitter, or, conversely, from being bitter now becomes sweet, it will be said to undergo motion in respect to flavour
; to both of these instances, as well as to those previously mentioned, we shall apply the term qualitative motion
. And further, it is not only things which are altered in regard to colour and flavour which, we say, undergo motion; when a warm thing becomes cold, and a cold warm, here too we speak of its undergoing motion; similarly also when any-