opposite which does not survive is described indifferently in both
ways, 'becoming that from this' or 'this becoming that'. We say both
that 'the unmusical becomes musical', and that 'from unmusical he
becomes musical'. And so both forms are used of the complex, 'becoming
a musical man from an unmusical man', and unmusical man becoming a
musical man'.

  But there are different senses of 'coming to be'. In some cases we
do not use the expression 'come to be', but 'come to be so-and-so'.
Only substances are said to 'come to be' in the unqualified sense.

  Now in all cases other than substance it is plain that there must be
some subject, namely, that which becomes. For we know that when a
thing comes to be of such a quantity or quality or in such a relation,
time, or place, a subject is always presupposed, since substance alone
is not predicated of another subject, but everything else of
substance.

  But that substances too, and anything else that can be said 'to
be' without qualification, come to be from some substratum, will
appear on examination. For we find in every case something that
underlies from which proceeds that which comes to be; for instance,
animals and plants from seed.

  Generally things which come to be, come to be in different ways: (1)
by change of shape, as a statue; (2) by addition, as things which
grow; (3) by taking away, as the Hermes from the stone; (4) by putting
together, as a house; (5) by alteration, as things which 'turn' in
respect of their material substance.

  It is plain that these are all cases of coming to be from a
substratum.

  Thus, clearly, from what has been said, whatever comes to be is
always complex. There is, on the one hand, (a) something which comes
into existence, and again (b) something which becomes that-the
latter (b) in two senses, either the subject or the opposite. By the
'opposite' I mean the 'unmusical', by the 'subject' 'man', and
similarly I call the absence of shape or form or order the 'opposite',
and the bronze or stone or gold the 'subject'.

  Plainly then, if there are conditions and principles which
constitute natural objects and from which they primarily are or have
come to be-have come to be, I mean, what each is said to be in its
essential nature, not what each is in respect of a concomitant
attribute-plainly, I say, everything comes to be from both subject and
form. For 'musical man' is composed (in a way) of 'man' and 'musical':
you can analyse it into the definitions of its elements. It is clear
then that what comes to be will come to be from these elements.

  Now the subject is one numerically, though it is two in form. (For
it is the man, the gold-the 'matter' generally-that is counted, for it
is more of the nature of a 'this', and what comes to be does not
come from it in virtue of a concomitant attribute; the privation, on
the other hand, and the contrary are incidental in the process.) And
the positive form is one-the order, the acquired art of music, or
any similar predicate.

  There is a sense, therefore, in which we must declare the principles
to be two, and a sense in which they are three; a sense in which the
contraries are the principles-say for example the musical and the
unmusical, the hot and the cold, the tuned and the untuned-and a sense
in which they are not, since it is impossible for the contraries to be
acted on by each other. But this difficulty also is solved by the fact
that the substratum is different from the contraries, for it is itself
not a contrary. The principles therefore are, in a way, not more in
number than the contraries, but as it were two, nor yet precisely two,
since there is a difference of essential nature, but three. For 'to be
man' is different from 'to be unmusical', and 'to be unformed' from
'to be bronze'.

  We have now stated the number of the principles of natural objects
which are subject to generation, and how the number is reached: and it
is clear that there must be a substratum for the contraries, and
that the contraries must be two. (Yet in another way of putting it
this is not necessary, as one of the contraries will serve to effect
the change by its successive absence and presence.)

  The underlying nature is an object of scientific knowledge, by an
analogy. For as the bronze is to the statue, the wood to the bed, or
the matter and the formless before receiving form to any thing which
has form, so is the underlying nature to substance, i.e. the 'this' or
existent.

  This then is one principle (though not one or existent in the same
sense as the 'this'), and the definition was one as we agreed; then
further there is its contrary, the privation. In what sense these
are two, and in what sense more, has been stated above. Briefly, we
explained first that only the contraries were principles, and later
that a substratum was indispensable, and that the principles were
three; our last statement has elucidated the difference between the
contraries, the mutual relation of the principles, and the nature of
the substratum. Whether the form or the substratum is the essential
nature of a physical object is not yet clear. But that the
principles are three, and in what sense, and the way in which each
is a principle, is clear.

  So much then for the question of the number and the nature of the
principles.