GREEK PHILOSOPHY - II.
Physics, or the Philosophy of Nature
We have just seen that substance is
immovable and movable, and that the science of immovable substance, or of
substance as immovable, is First Philosophy or Metaphysics. The science
of movable substance, or of substance as movable, is Physics, or the
Philosophy of Nature. Movable substance is of two kinds: that which has, in a
manner, the principle of motion in itself, and that which has not. But the
principle of motion in this is the soul; hence Physics discusses, and is
primarily the philosophy of, the soul(1).
Essential Character of Nature
Nature, as having the principle of motion
within itself, is possessed of a soul,
is a living being, and its works are in all respects like those of an artist,
except that the latter have their efficient cause outside themselves, whereas
the efficient cause of the works of nature is immanent. Nature is governed by
the principle of the end and does nothing in vain(2). The end is an immanent end:
the end of the plant or the animal is to be just the plant or the animal.
Nature is both matter and form, but the form prevails to such an extent that
nature works generally, if not always, in the same way and towards
cognizable ends. There is, indeed, a certain mechanical necessity in
nature: but it is secondary, not primary, a condition merely, not a
cause,—just as "heavy" and "light" are conditions but not causes with
reference to the house made by the builder(3).
There is, also, a certain element of contingency in nature: there are
in the animal kingdom monstrosities, which are examples of nature's failure to
attain to form, or reason. Such failures are inherent in matter, which is the
contingent cause of what is accidental(4). But in spite of necessity and chance, or
contingency, nature is governed by form, or inherent end.
Method of the Philosophy of Nature
As governed by the end, or reason, nature
is an object of science ; and yet owing to the contingency inherent in matter,
the science of nature, or Physics, is not so purely a demonstrative science as
is that of being. Careful and comprehensive observation and induction are
requisite as a basis from which to rise to principles ; truth is not to be
attained by those who, preoccupied with theories, neglect
facts(5). But observations and deductions from facts are to be governed by the
idea of the end: the highest of the "causes" of knowledge as of being is in Physics, as in Metaphysics, the final cause.
Motion, Space, and Time
Motion (κίνησις) is the entelechy or natural state of
the potential as potential. In other words the world of matter is inherently a
world of movement—matter has reality (form) for us only as in motion. Motion
is distinguished by Aristotle from change (μεταβολή), which embraces
origin and decay, increase and diminution, alteration (in kind or quality), and
change in place. Motion is merely a kind of change, and includes only the six
last-mentioned kinds of change, all (six) of which are or involve change in
place. The six kinds of motion referred to may be grouped into three :
changes in quality (alteration), in quantity (increase and diminution),
and in "place". "Place'' (τόπος) is not (as we understand it) position, nor
the space occupied by a body, but the limit presented to a body by a surrounding
body or by surrounding bodies; it may be compared to a vessel in which water or
any other material substance is held. No "place" is empty (there is no empty
space); the world is a plenum. The movement of bodies is therefore merely an
exchanging of "places". Space is not infinite but ends with the sphere of the
fixed stars. The world as a whole is not in any "place". The perfect motion is
circular; for only such a motion, a motion the path of which is without
beginning or end, answers to the eternal nature of the Prime Mover. Such is the
motion of the sphere of the fixed
stars, upon which God acts, though without touching it. Motion is eternal, since
every motion of a real thing implies, on the one hand, an antecedent motion
which, again, implies another and so on in infinitum and, on the other hand, a
subsequent one, which, in turn, implies another and so on in infinitum. The
eternity of motion implies an eternal cause of motion(6), —a corollary to the
theorem of the eternity of motion is that of the eternity of time. Time is the "number of motion with reference to earlier and later". We should have no
conception of time merely from the idea of a "now". Consciousness of succession
(arising from the perception of motion) is also necessary. Practically, however,
every "now" is a union of before" and "after," and so time is in itself potentially
infinite. Time as a numbering presupposes a "numberer," infinite time an
infinite mind(7). The universe has always been, and always will be the same.
The Visible Universe
The visible universe was conceived by Aristotle as a
living sphere. Exterior to the sphere is the abode of the Prime Mover. That
part of the sphere nearest the abode of the Prime Mover—the region of the fixed stars—partakes of the perfection of the Prime Mover, or Deity, dwelling
in felicity and realizing the highest end of existence; the centre of the
sphere, the region of our earth, is the
place of imperfection. The region of the planets is intermediate in character as
in place between the other two. The material
elements are five in number,—earth, water, air, fire, ether. Ether, the most
perfect of them, exists only in the upper heaven, is not subject
to changes either in quality or in quantity, but to change in place only, and
has only a circular (perfect) motion. Of the other elements, earth is lowest,
fire highest, in place and nature. They easily pass into one another, being active
and passive in nature, and are, as compared with ether, the fifth element, or
quintessence, imperfect and the cause of imperfection in the lower world. Their
motions are not circular : earth moves downwards, fire upwards, air and water
having intermediate motions. Of fire, air, water, and earth all living beings
are composed, homogeneous parts, e.g., flesh or bones, being formed of like
parts, and heterogeneous of the homogeneous.
Graduated Scale of Being in Nature
There is throughout nature a gradation of
being, and at certain points it is with difficulty that beings of one kind can
be distinguished from those of another(8). Certain plants (e.g., the sponge) very
closely resemble animals, and the attributes possessed by animals are possessed
by man in a higher degree of perfection. Life pervades even the elements.
(1) Metaphysics, Bk. V. ch. I; Bk. X. ch. 3, etc.
(2) De Anima, Bk. III. ch. 12.
(3) Physics, Bk. II. 9.
(4) Metaphysics, Bk. V. ch. 7.
(5) Posterior Analytics, Bk. I. ch. 33; De Generatione et Corruptione, Bk. I. ch.
(6) Physics, Bk. VIII. ch. I.
(7) Ibid., Bk. VIII. ch. I; Bk. VI. ch. 6, etc.
(8) On the Parts of Animals, Bk. IV. ch. 5.