It is clear then that there are causes, and that the number of
them is what we have stated. The number is the same as that of the
things comprehended under the question "why". The "why" is referred
ultimately either (1), in things which do not involve motion, e.g.
in mathematics, to the "what" (to the definition of "straight line" or
"commensurable", &c.), or (2) to what initiated a motion, e.g. "why
did they go to war?-because there had been a raid"; or (3) we are
inquiring "for the sake of what?"-"that they may rule"; or (4), in the
case of things that come into being, we are looking for the matter.
The causes, therefore, are these and so many in number.

    Now, the causes being four, it is the business of the physicist to
know about them all, and if he refers his problems back to all of
them, he will assign the "why" in the way proper to his science-the
matter, the form, the mover, "that for the sake of which". The last
three often coincide; for the "what" and "that for the sake of
which" are one, while the primary source of motion is the same in
species as these (for man generates man), and so too, in general,
are all things which cause movement by being themselves moved; and
such as are not of this kind are no longer inside the province of
physics, for they cause motion not by possessing motion or a source of
motion in themselves, but being themselves incapable of motion.
Hence there are three branches of study, one of things which are
incapable of motion, the second of things in motion, but
indestructible, the third of destructible things.

    The question "why", then, is answered by reference to the matter, to
the form, and to the primary moving cause. For in respect of coming to
be it is mostly in this last way that causes are investigated-"what
comes to be after what? what was the primary agent or patient?" and so
at each step of the series.

    Now the principles which cause motion in a physical way are two,
of which one is not physical, as it has no principle of motion in
itself. Of this kind is whatever causes movement, not being itself
moved, such as (1) that which is completely unchangeable, the
primary reality, and (2) the essence of that which is coming to be,
i.e. the form; for this is the end or "that for the sake of which".
Hence since nature is for the sake of something, we must know this
cause also. We must explain the "why" in all the senses of the term,
namely, (1) that from this that will necessarily result ("from this"
either without qualification or in most cases); (2) that "this must be
so if that is to be so" (as the conclusion presupposes the premisses);
(3) that this was the essence of the thing; and (4) because it is
better thus (not without qualification, but with reference to the
essential nature in each case).