UNITY OF SUBJECT MATTER
“Let no man,” says Bacon, “look for much progress
in the sciences... unless natural philosophy be carried
on and applied to particular sciences and particular
sciences be carried back again to natural philosophy.”
This holds not only for “astronomy, optics, music, a
number of mechanical arts, medicine itself,” but also
for “what one might more wonder at, moral and polit-
ical philosophy, and the logical sciences” (
Novum
organum, Book II, Aph. lxxx,
Works, VIII, 112). Bacon
does not regard these other sciences as parts of natural
philosophy, but as having their “roots” in it. Natural
philosophy itself, however, contains within it certain
constituent sciences which form a pyramid. These are
distinguished from one another not by their subjects,
but by their levels of generality in knowledge of the
one subject, nature. At the base of the pyramid is
natural history. On that is built physics, which has two
parts, one less general and one more general. On
physics is built metaphysics (a science distinct from
first philosophy) which brings the axioms of physics
under still more general axioms. At the vertical point
of the pyramid is “the summary law of nature”—i.e.,
“the force implanted by God in the first particles of
matter from the multiplication whereof all the variety
of things proceeds and is made up” (
De principiis,
Works, X, 345), though knowledge of it is probably
beyond the reach of the human mind. The three
inductively ordered levels, natural history, physics, and
metaphysics, are “the true stages of knowledge.” Given
the one subject, nature, it is Bacon's logic of induction
which alone determines both the divisions and the
unity of the sciences which comprise natural philoso-
phy. Bacon also divided all human learning on the basis
of Memory (History), Imagination (Poetry), and Reason
(Philosophy).
Hobbes's materialism taken in conjunction with his
logic or method has similar reductive consequences for
the division and unity of the sciences, though the
method is now deductive, not inductive. As knowledge
of causes, science is in every case knowledge of the
motions of bodies by which effects are generated. These
motions are the single subject of all sciences, one sci-
ence being distinguished from another only by the
complexity of the motions which it investigates. Thus
after first philosophy—which consists of definitions of
the most general names—comes geometry, the science
of the simple motions of a body by which lines, sur-
faces, and figures are produced. Then follows the sci-
ence concerned with the effects of the impact of
moving bodies; then the science of the effects of the
internal motions of bodies, that is, physics or the study
of sensible phenomena such as light, colors, sounds,
tastes, odors, heat, etc., and of the senses themselves;
then moral philosophy or the science of the motions
of the mind such as appetite, aversion, love,
benevolence, etc., for these are motions consequent
upon the motions of sense; and finally, because the
motions of the mind are the causes of the common-
wealth, there comes civil or political philosophy (Con-
cerning Body, Ch. VI, Secs. 6, 7).
For Spinoza and Leibniz also the sciences are not
diversified by the kinds of beings which they investi-
gate. There is for Spinoza only one substance, God or
nature, and for Leibniz only one kind of substance,
monads. In Spinoza's world the essence of any individ-
ual finite thing is the power or effort by which it
endeavors to persevere in its being. This conatus fol-
lows from, or is a mode of, the infinite power of God,
a power which can be expressed to the perceiving
intellect either as infinite extension or as infinite
thought (Ethics, Part I, definitions 4, 6). Corre-
spondingly, the conatus constituting the essence of the
individual thing can be expressed either as body or as
mind. Thus physics on the one hand and the science
of the thoughts and emotions of the mind on the other
are knowledge merely of the same conatus, but as
differently expressed, and the order of causes in the
one science will be identical with the order of causes
in the other (ibid., Part II, prop. 7). Spinoza's proposed
aim of acquiring “the knowledge of the union which
the mind has with the whole of nature” required the
study of Moral Philosophy, the Theory of the Educa-
tion of Children, the science of Medicine and the art
of Mechanics (De emendatione, II, 13-15). Leibniz too
adopts a double aspect conception, but in his case it
is used for correlating metaphysics, the science of
beings as they are in themselves, that is, as indivisible
spiritual substances, with physics, the science of these
same beings as they appear, that is, as material
phenomena or extended masses. “These two realms are
distinct, each one being governed by its own laws....
But the two very different series are in mutual corre-
spondence in the same corporeal substance and
harmonize so perfectly that it is just as if one were
ruled by the influence of the other” (Loemker, p. 675).
