All thinkers then agree in making the contraries principles, both
those who describe the All as one and unmoved (for even Parmenides
treats hot and cold as principles under the names of fire and earth)
and those too who use the rare and the dense. The same is true of
Democritus also, with his plenum and void, both of which exist, be
says, the one as being, the other as not-being. Again he speaks of
differences in position, shape, and order, and these are genera of
which the species are contraries, namely, of position, above and
below, before and behind; of shape, angular and angle-less, straight
and round.

    It is plain then that they all in one way or another identify the
contraries with the principles. And with good reason. For first
principles must not be derived from one another nor from anything
else, while everything has to be derived from them. But these
conditions are fulfilled by the primary contraries, which are not
derived from anything else because they are primary, nor from each
other because they are contraries.

    But we must see how this can be arrived at as a reasoned result,
as well as in the way just indicated.

    Our first presupposition must be that in nature nothing acts on,
or is acted on by, any other thing at random, nor may anything come
from anything else, unless we mean that it does so in virtue of a
concomitant attribute. For how could "white" come from "musical",
unless "musical" happened to be an attribute of the not-white or of
the black? No, "white" comes from "not-white"-and not from any
"not-white", but from black or some intermediate colour. Similarly,
"musical" comes to be from "not-musical", but not from any thing other
than musical, but from "unmusical" or any intermediate state there may
be.

    Nor again do things pass into the first chance thing; "white" does
not pass into "musical" (except, it may be, in virtue of a concomitant
attribute), but into "not-white"-and not into any chance thing which
is not white, but into black or an intermediate colour; "musical"
passes into "not-musical"-and not into any chance thing other than
musical, but into "unmusical" or any intermediate state there may be.

    The same holds of other things also: even things which are not
simple but complex follow the same principle, but the opposite state
has not received a name, so we fail to notice the fact. What is in
tune must come from what is not in tune, and vice versa; the tuned
passes into untunedness-and not into any untunedness, but into the
corresponding opposite. It does not matter whether we take attunement,
order, or composition for our illustration; the principle is obviously
the same in all, and in fact applies equally to the production of a
house, a statue, or any other complex. A house comes from certain
things in a certain state of separation instead of conjunction, a
statue (or any other thing that has been shaped) from
shapelessness-each of these objects being partly order and partly
composition.

    If then this is true, everything that comes to be or passes away
from, or passes into, its contrary or an intermediate state. But the
intermediates are derived from the contraries-colours, for instance,
from black and white. Everything, therefore, that comes to be by a
natural process is either a contrary or a product of contraries.

    Up to this point we have practically had most of the other writers
on the subject with us, as I have said already: for all of them
identify their elements, and what they call their principles, with the
contraries, giving no reason indeed for the theory, but contrained
as it were by the truth itself. They differ, however, from one another
in that some assume contraries which are more primary, others
contraries which are less so: some those more knowable in the order of
explanation, others those more familiar to sense. For some make hot
and cold, or again moist and dry, the conditions of becoming; while
others make odd and even, or again Love and Strife; and these differ
from each other in the way mentioned.

    Hence their principles are in one sense the same, in another
different; different certainly, as indeed most people think, but the
same inasmuch as they are analogous; for all are taken from the same
table of columns, some of the pairs being wider, others narrower in
extent. In this way then their theories are both the same and
different, some better, some worse; some, as I have said, take as
their contraries what is more knowable in the order of explanation,
others what is more familiar to sense. (The universal is more knowable
in the order of explanation, the particular in the order of sense: for
explanation has to do with the universal, sense with the
particular.) "The great and the small", for example, belong to the
former class, "the dense and the rare" to the latter.

    It is clear then that our principles must be contraries.