Prologue — Prologue

1. As the Philosopher says in On the Soul III, the sciences are divided off in the same manner as things are—for all habits are distinguished by their objects, from which they are specified. Now the things considered by natural science are motion and mobile being. Thus the Philosopher says in Physics II that whatever things move, they themselves being moved, these belong to physical speculation. Consequently, it is according to the differences between motions and mobiles that the parts of natural science must be distinguished and ordered.

Now the first motion is local motion, which is more perfect than the other kinds, and common to all natural bodies, as is proved in Physics VII. Therefore, after the study of motions and mobiles in common in the book of the Physics, it was first necessary to treat of bodies as they are moved with local motion. This was in the book On the Heavens, which is the second book of natural science. What remains, therefore, is to consider the other subsequent motions which are not common to all bodies but are found only in lower bodies.

Among these motions, generation and corruption obtain the primacy. For alteration is directed to generation as to its end, and the end is by nature more perfect than what leads to it. Growth, likewise, is subsequent to generation, for growth does not take place without a certain particular generation, namely, that by which food is converted into the thing fed. Thus the Philosopher says in On the Soul II that food nourishes in so far as it is potentially flesh, but it produces increase inasmuch as potentially it is quantified flesh. Therefore, since these motions are in a certain way consequent upon generation, they must be studied along with generation and corruption.

2. Now it should be noted that whatever is found in a number of things should first be considered in common before coming to the specific cases. Otherwise the same thing will be frequently repeated, in that what is common will be repeated in each individual case, as the Philosopher proves in On the Parts of Animals I. Consequently, generation and corruption should be considered in common before coming to the parts [i.e., species] thereof. Likewise, it should be noted that if in any genus there be found some first thing which is the cause of the other things in that genus, the study of the common genus and of that which is first in that genus will belong to the same study. For that first thing is the cause of the entire genus, and anyone who studies some genus must consider the causes of the entire genus. That is why the Philosopher in the Metaphysics at once studies being in general and first being, which is separated from matter. Now in the genus of generable and corruptible things there are found certain first principles, namely, the elements, which are the cause of generation and corruption and alteration in all other bodies. Hence Aristotle in this book, which is the third part of natural science, discusses not only generation and corruption in general and other consequent motions, but also generation and corruption of the elements.

With these prefatory remarks to show Aristotle's intention in this book, we now arrive at its exposition.