manifested in eternal phenomena; and there is hypothetical
necessity, manifested in everything that is generated by nature as
in everything that is produced by art, be it a house or what it may.
For if a house or other such final object is to be realized, it is
necessary that such and such material shall exist; and it is necessary
that first this then that shall be produced, and first this and then
that set in motion, and so on in continuous succession, until the
end and final result is reached, for the sake of which each prior
thing is produced and exists. As with these productions of art, so
also is it with the productions of nature. The mode of necessity,
however, and the mode of ratiocination are different in natural
science from what they are in the theoretical sciences; of which we
have spoken elsewhere. For in the latter the starting-point is that
which is; in the former that which is to be. For it is that which is
yet to be-health, let us say, or a man-that, owing to its being of
such and such characters, necessitates the pre-existence or previous
production of this and that antecedent; and not this or that
antecedent which, because it exists or has been generated, makes it
necessary that health or a man is in, or shall come into, existence.
Nor is it possible to track back the series of necessary antecedents
to a starting-point, of which you can say that, existing itself from
eternity, it has determined their existence as its consequent. These
however again, are matters that have been dealt with in another
treatise. There too it was stated in what cases absolute and
hypothetical necessity exist; in what cases also the proposition
expressing hypothetical necessity is simply convertible, and what
cause it is that determines this convertibility.
Another matter which must not be passed over without consideration
is, whether the proper subject of our exposition is that with which
the ancient writers concerned themselves, namely, what is the
process of formation of each animal; or whether it is not rather, what
are the characters of a given creature when formed. For there is no
small difference between these two views. The best course appears to
be that we should follow the method already mentioned, and begin
with the phenomena presented by each group of animals, and, when
this is done, proceed afterwards to state the causes of those
phenomena, and to deal with their evolution. For elsewhere, as for
instance in house building, this is the true sequence. The plan of the
house, or the house, has this and that form; and because it has this
and that form, therefore is its construction carried out in this or
that manner. For the process of evolution is for the sake of the thing
Anally evolved, and not this for the sake of the process.
Empedocles, then, was in error when he said that many of the
characters presented by animals were merely the results of
incidental occurrences during their development; for instance, that
the backbone was divided as it is into vertebrae, because it
happened to be broken owing to the contorted position of the foetus in
the womb. In so saying he overlooked the fact that propagation implies
a creative seed endowed with certain formative properties. Secondly,
he neglected another fact, namely, that the parent animal
pre-exists, not only in idea, but actually in time. For man is
generated from man; and thus it is the possession of certain
characters by the parent that determines the development of like
characters in the child. The same statement holds good also for the
operations of art, and even for those which are apparently
spontaneous. For the same result as is produced by art may occur
spontaneously. Spontaneity, for instance, may bring about the
restoration of health. The products of art, however, require the
pre-existence of an efficient cause homogeneous with themselves,
such as the statuary's art, which must necessarily precede the statue;
for this cannot possibly be produced spontaneously. Art indeed
consists in the conception of the result to be produced before its
realization in the material. As with spontaneity, so with chance;
for this also produces the same result as art, and by the same
process.
The fittest mode, then, of treatment is to say, a man has such and
such parts, because the conception of a man includes their presence,
and because they are necessary conditions of his existence, or, if
we cannot quite say this, which would be best of all, then the next
thing to it, namely, that it is either quite impossible for him to
exist without them, or, at any rate, that it is better for him that
they should be there; and their existence involves the existence of
other antecedents. Thus we should say, because man is an animal with
such and such characters, therefore is the process of his
development necessarily such as it is; and therefore is it
accomplished in such and such an order, this part being formed
first, that next, and so on in succession; and after a like fashion
should we explain the evolution of all other works of nature.
Now that with which the ancient writers, who first philosophized
about Nature, busied themselves, was the material principle and the
material cause. They inquired what this is, and what its character;
how the universe is generated out of it, and by what motor
influence, whether, for instance, by antagonism or friendship, whether
by intelligence or spontaneous action, the substratum of matter
being assumed to have certain inseparable properties; fire, for
instance, to have a hot nature, earth a cold one; the former to be
light, the latter heavy. For even the genesis of the universe is
thus explained by them. After a like fashion do they deal also with
the development of plants and of animals. They say, for instance, that
the water contained in the body causes by its currents the formation
of the stomach and the other receptacles of food or of excretion;
and that the breath by its passage breaks open the outlets of the
nostrils; air and water being the materials of which bodies are
made; for all represent nature as composed of such or similar
substances.
But if men and animals and their several parts are natural
phenomena, then the natural philosopher must take into consideration
not merely the ultimate substances of which they are made, but also
flesh, bone, blood, and all other homogeneous parts; not only these,
but also the heterogeneous parts, such as face, hand, foot; and must
examine how each of these comes to be what it is, and in virtue of
what force. For to say what are the ultimate substances out of which
an animal is formed, to state, for instance, that it is made of fire
or earth, is no more sufficient than would be a similar account in the
case of a couch or the like. For we should not be content with
saying that the couch was made of bronze or wood or whatever it
might be, but should try to describe its design or mode of composition
in p
reference to the material; or, if we did deal with the material,
it would at any rate be with the concretion of material and form.
For a couch is such and such a form embodied in this or that matter,
or such and such a matter with this or that form; so that its shape
and structure must be included in our description. For the formal
nature is of greater importance than the material nature.
Does, then, configuration and colour constitute the essence of the
various animals and of their several parts? For if so, what Democritus
says will be strictly correct. For such appears to have been his
notion. At any rate he says that it is evident to every one what
form it is that makes the man, seeing that he is recognizable by his
shape and colour. And yet a dead body has exactly the same
configuration as a living one; but for all that is not a man. So
also no hand of bronze or wood or constituted in any but the
appropriate way can possibly be a hand in more than name. For like a
physician in a painting, or like a flute in a sculpture, in spite of
its name it will be unable to do the office which that name implies.
Precisely in the same way no part of a dead body, such I mean as its
eye or its hand, is really an eye or a hand. To say, then, that
shape and colour constitute the animal is an inadequate statement, and
is much the same as if a woodcarver were to insist that the hand he
had cut out was really a hand. Yet the physiologists, when they give
an account of the development and causes of the animal form, speak
very much like such a craftsman. What, however, I would ask, are the
forces by which the hand or the body was fashioned into its shape? The
woodcarver will perhaps say, by the axe or the auger; the
physiologist, by air and by earth. Of these two answers the
artificer's is the better, but it is nevertheless insufficient. For it
is not enough for him to say that by the stroke of his tool this
part was formed into a concavity, that into a flat surface; but he
must state the reasons why he struck his blow in such a way as to
effect this, and what his final object was; namely, that the piece
of wood should develop eventually into this or that shape. It is
plain, then, that the teaching of the old physiologists is inadequate,
and that the true method is to state what the definitive characters
are that distinguish the animal as a whole; to explain what it is both
in substance and in form, and to deal after the same fashion with
its several organs; in fact, to proceed in exactly the same way as
we should do, were we giving a complete description of a couch.
If now this something that constitutes the form of the living
being be the soul, or part of the soul, or something that without
the soul cannot exist; as would seem to be the case, seeing at any
rate that when the soul departs, what is left is no longer a living
animal, and that none of the parts remain what they were before,
excepting in mere configuration, like the animals that in the fable
are turned into stone; if, I say, this be so, then it will come within
the province of the natural philosopher to inform himself concerning
the soul, and to treat of it, either in its entirety, or, at any rate,
of that part of it which constitutes the essential character of an
animal; and it will be his duty to say what this soul or this part
of a soul is; and to discuss the attributes that attach to this
essential character, especially as nature is spoken of in two
senses, and the nature of a thing is either its matter or its essence;
nature as essence including both the motor cause and the final
cause. Now it is in the latter of these two senses that either the
whole soul or some part of it constitutes the nature of an animal; and
inasmuch as it is the presence of the soul that enables matter to
constitute the animal nature, much more than it is the presence of
matter which so enables the soul, the inquirer into nature is bound on
every ground to treat of the soul rather than of the matter. For
though the wood of which they are made constitutes the couch and the
tripod, it only does so because it is capable of receiving such and
such a form.
What has been said suggests the question, whether it is the whole
soul or only some part of it, the consideration of which comes
within the province of natural science. Now if it be of the whole soul
that this should treat, then there is no place for any other
philosophy beside it. For as it belongs in all cases to one and the
same science to deal with correlated subjects-one and the same
science, for instance, deals with sensation and with the objects of
sense-and as therefore the intelligent soul and the objects of
intellect, being correlated, must belong to one and the same
science, it follows that natural science will have to include the
whole universe in its province. But perhaps it is not the whole
soul, nor all its parts collectively, that constitutes the source of
motion; but there may be one part, identical with that in plants,
which is the source of growth, another, namely the sensory part, which
is the source of change of quality, while still another, and this
not the intellectual part, is the source of locomotion. I say not
the intellectual part; for other animals than man have the power of
locomotion, but in none but him is there intellect. Thus then it is
plain that it is not of the whole soul that we have to treat. For it
is not the whole soul that constitutes the animal nature, but only
some part or parts of it. Moreover, it is impossible that any
abstraction can form a subject of natural science, seeing that
everything that Nature makes is means to an end. For just as human
creations are the products of art, so living objects are manifest in
the products of an analogous cause or principle, not external but
internal, derived like the hot and the cold from the environing
universe. And that the heaven, if it had an origin, was evolved and is
maintained by such a cause, there is therefore even more reason to
believe, than that mortal animals so originated. For order and
definiteness are much more plainly manifest in the celestial bodies
than in our own frame; while change and chance are characteristic of
the perishable things of earth. Yet there are some who, while they
allow that every animal exists and was generated by nature,
nevertheless hold that the heaven was constructed to be what it is
by chance and spontaneity; the heaven, in which not the faintest
sign of haphazard or of disorder is discernible! Again, whenever there
is plainly some final end, to which a motion tends should nothing
stand in the way, we always say that such final end is the aim or
purpose of the motion; and from this it is evident that there must
be a something or other really existing, corresponding to what we call
by the name of Nature. For a given germ does not give rise to any
chance living being, nor spring from any chance one; but each germ
springs from a definite parent and gives rise to a definite progen
y.
And thus it is the germ that is the ruling influence and fabricator of
the offspring. For these it is by nature, the offspring being at any
rate that which in nature will spring from it. At the same time the
offspring is anterior to the germ; for germ and perfected progeny
are related as the developmental process and the result. Anterior,
however, to both germ and product is the organism from which the
germ was derived. For every germ implies two organisms, the parent and
the progeny. For germ or seed is both the seed of the organism from
which it came, of the horse, for instance, from which it was
derived, and the seed of the organism that will eventually arise
from it, of the mule, for example, which is developed from the seed of
the horse. The same seed then is the seed both of the horse and of the
mule, though in different ways as here set forth. Moreover, the seed
is potentially that which will spring from it, and the relation of
potentiality to actuality we know.
There are then two causes, namely, necessity and the final end.
For many things are produced, simply as the results of necessity. It
may, however, be asked, of what mode of necessity are we speaking when
we say this. For it can be of neither of those two modes which are set
forth in the philosophical treatises. There is, however, the third
mode, in such things at any rate as are generated. For instance, we
say that food is necessary; because an animal cannot possibly do
without it. This third mode is what may be called hypothetical
necessity. Here is another example of it. If a piece of wood is to
be split with an axe, the axe must of necessity be hard; and, if hard,
must of necessity be made of bronze or iron. Now exactly in the same
way the body, which like the axe is an instrument-for both the body as
a whole and its several parts individually have definite operations
for which they are made-just in the same way, I say, the body, if it
is to do its work, must of necessity be of such and such a
character, and made of such and such materials.
It is plain then that there are two modes of causation, and that
both of these must, so far as possible, be taken into account in