incidental attribute, whereas the real nature is the other, which,
further, persists continuously through the process of making.

  But if the material of each of these objects has itself the same
relation to something else, say bronze (or gold) to water, bones (or
wood) to earth and so on, that (they say) would be their nature and
essence. Consequently some assert earth, others fire or air or water
or some or all of these, to be the nature of the things that are.
For whatever any one of them supposed to have this character-whether
one thing or more than one thing-this or these he declared to be the
whole of substance, all else being its affections, states, or
dispositions. Every such thing they held to be eternal (for it could
not pass into anything else), but other things to come into being
and cease to be times without number.

  This then is one account of 'nature', namely that it is the
immediate material substratum of things which have in themselves a
principle of motion or change.

  Another account is that 'nature' is the shape or form which is
specified in the definition of the thing.

  For the word 'nature' is applied to what is according to nature
and the natural in the same way as 'art' is applied to what is
artistic or a work of art. We should not say in the latter case that
there is anything artistic about a thing, if it is a bed only
potentially, not yet having the form of a bed; nor should we call it a
work of art. The same is true of natural compounds. What is
potentially flesh or bone has not yet its own 'nature', and does not
exist until it receives the form specified in the definition, which we
name in defining what flesh or bone is. Thus in the second sense of
'nature' it would be the shape or form (not separable except in
statement) of things which have in themselves a source of motion. (The
combination of the two, e.g. man, is not 'nature' but 'by nature' or
'natural'.)

  The form indeed is 'nature' rather than the matter; for a thing is
more properly said to be what it is when it has attained to fulfilment
than when it exists potentially. Again man is born from man, but not
bed from bed. That is why people say that the figure is not the nature
of a bed, but the wood is-if the bed sprouted not a bed but wood would
come up. But even if the figure is art, then on the same principle the
shape of man is his nature. For man is born from man.

  We also speak of a thing's nature as being exhibited in the
process of growth by which its nature is attained. The 'nature' in
this sense is not like 'doctoring', which leads not to the art of
doctoring but to health. Doctoring must start from the art, not lead
to it. But it is not in this way that nature (in the one sense) is
related to nature (in the other). What grows qua growing grows from
something into something. Into what then does it grow? Not into that
from which it arose but into that to which it tends. The shape then is
nature.

  'Shape' and 'nature', it should be added, are in two senses. For the
privation too is in a way form. But whether in unqualified coming to
be there is privation, i.e. a contrary to what comes to be, we must
consider later.

                                 2

  We have distinguished, then, the different ways in which the term
'nature' is used.

  The next point to consider is how the mathematician differs from the
physicist. Obviously physical bodies contain surfaces and volumes,
lines and points, and these are the subject-matter of mathematics.

  Further, is astronomy different from physics or a department of
it? It seems absurd that the physicist should be supposed to know
the nature of sun or moon, but not to know any of their essential
attributes, particularly as the writers on physics obviously do
discuss their shape also and whether the earth and the world are
spherical or not.

  Now the mathematician, though he too treats of these things,
nevertheless does not treat of them as the limits of a physical
body; nor does he consider the attributes indicated as the
attributes of such bodies. That is why he separates them; for in
thought they are separable from motion, and it makes no difference,
nor does any falsity result, if they are separated. The holders of the
theory of Forms do the same, though they are not aware of it; for they
separate the objects of physics, which are less separable than those
of mathematics. This becomes plain if one tries to state in each of
the two cases the definitions of the things and of their attributes.
'Odd' and 'even', 'straight' and 'curved', and likewise 'number',
'line', and 'figure', do not involve motion; not so 'flesh' and 'bone'
and 'man'-these are defined like 'snub nose', not like 'curved'.

  Similar evidence is supplied by the more physical of the branches of
mathematics, such as optics, harmonics, and astronomy. These are in
a way the converse of geometry. While geometry investigates physical
lines but not qua physical, optics investigates mathematical lines,
but qua physical, not qua mathematical.

  Since 'nature' has two senses, the form and the matter, we must
investigate its objects as we would the essence of snubness. That
is, such things are neither independent of matter nor can be defined
in terms of matter only. Here too indeed one might raise a difficulty.
Since there are two natures, with which is the physicist concerned? Or