The physicists on the other hand have two modes of explanation.

    The first set make the underlying body one either one of the three
or something else which is denser than fire and rarer than air then
generate everything else from this, and obtain multiplicity by
condensation and rarefaction. Now these are contraries, which may be
generalized into "excess and defect". (Compare Plato's "Great and
Small"-except that he make these his matter, the one his form, while
the others treat the one which underlies as matter and the
contraries as differentiae, i.e. forms).

    The second set assert that the contrarieties are contained in the
one and emerge from it by segregation, for example Anaximander and
also all those who assert that "what is" is one and many, like
Empedocles and Anaxagoras; for they too produce other things from
their mixture by segregation. These differ, however, from each other
in that the former imagines a cycle of such changes, the latter a
single series. Anaxagoras again made both his "homceomerous"
substances and his contraries infinite in multitude, whereas
Empedocles posits only the so-called elements.

    The theory of Anaxagoras that the principles are infinite in
multitude was probably due to his acceptance of the common opinion
of the physicists that nothing comes into being from not-being. For
this is the reason why they use the phrase "all things were
together" and the coming into being of such and such a kind of thing
is reduced to change of quality, while some spoke of combination and
separation. Moreover, the fact that the contraries proceed from each
other led them to the conclusion. The one, they reasoned, must have
already existed in the other; for since everything that comes into
being must arise either from what is or from what is not, and it is
impossible for it to arise from what is not (on this point all the
physicists agree), they thought that the truth of the alternative
necessarily followed, namely that things come into being out of
existent things, i.e. out of things already present, but imperceptible
to our senses because of the smallness of their bulk. So they assert
that everything has been mixed in every. thing, because they saw
everything arising out of everything. But things, as they say,
appear different from one another and receive different names
according to the nature of the particles which are numerically
predominant among the innumerable constituents of the mixture. For
nothing, they say, is purely and entirely white or black or sweet,
bone or flesh, but the nature of a thing is held to be that of which
it contains the most.

    Now (1) the infinite qua infinite is unknowable, so that what is
infinite in multitude or size is unknowable in quantity, and what is
infinite in variety of kind is unknowable in quality. But the
principles in question are infinite both in multitude and in kind.
Therefore it is impossible to know things which are composed of
them; for it is when we know the nature and quantity of its components
that we suppose we know a complex.

    Further (2) if the parts of a whole may be of any size in the
direction either of greatness or of smallness (by "parts" I mean
components into which a whole can be divided and which are actually
present in it), it is necessary that the whole thing itself may be
of any size. Clearly, therefore, since it is impossible for an
animal or plant to be indefinitely big or small, neither can its parts
be such, or the whole will be the same. But flesh, bone, and the
like are the parts of animals, and the fruits are the parts of plants.
Hence it is obvious that neither flesh, bone, nor any such thing can
be of indefinite size in the direction either of the greater or of the
less.

    Again (3) according to the theory all such things are already
present in one another and do not come into being but are constituents
which are separated out, and a thing receives its designation from its
chief constituent. Further, anything may come out of anything-water by
segregation from flesh and flesh from water. Hence, since every finite
body is exhausted by the repeated abstraction of a finite body, it
seems obviously to follow that everything cannot subsist in everything
else. For let flesh be extracted from water and again more flesh be
produced from the remainder by repeating the process of separation:
then, even though the quantity separated out will continually
decrease, still it will not fall below a certain magnitude. If,
therefore, the process comes to an end, everything will not be in
everything else (for there will be no flesh in the remaining water);
if on the other hand it does not, and further extraction is always
possible, there will be an infinite multitude of finite equal
particles in a finite quantity-which is impossible. Another proof
may be added: Since every body must diminish in size when something is
taken from it, and flesh is quantitatively definite in respect both of
greatness and smallness, it is clear that from the minimum quantity of
flesh no body can be separated out; for the flesh left would be less
than the minimum of flesh.

    Lastly (4) in each of his infinite bodies there would be already
present infinite flesh and blood and brain- having a distinct
existence, however, from one another, and no less real than the
infinite bodies, and each infinite: which is contrary to reason.

    The statement that complete separation never will take place is
correct enough, though Anaxagoras is not fully aware of what it means.
For affections are indeed inseparable. If then colours and states
had entered into the mixture, and if separation took place, there
would be a "white" or a "healthy" which was nothing but white or
healthy, i.e. was not the predicate of a subject. So his "Mind" is
an absurd person aiming at the impossible, if he is supposed to wish
to separate them, and it is impossible to do so, both in respect of
quantity and of quality- of quantity, because there is no minimum
magnitude, and of quality, because affections are inseparable.

    Nor is Anaxagoras right about the coming to be of homogeneous
bodies. It is true there is a sense in which clay is divided into
pieces of clay, but there is another in which it is not. Water and air
are, and are generated "from" each other, but not in the way in
which bricks come "from" a house and again a house "from" bricks;
and it is better to assume a smaller and finite number of
principles, as Empedocles does.