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Dictionary of the History of Ideas

Studies of Selected Pivotal Ideas
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Though it is customary to contrast Hume's empiri-
cism with the rationalism of Descartes and the Conti-
nental philosophers whom he influenced, their discus-
sions of causal explanation had certain important
features in common. One was concern about the notion
of force. Descartes and the occasionalists, attacking
what they took to be animistic conceptions of forces
in nature, raised questions about the origin of the idea
of force; and Nicolas Malebranche, employing argu-
ments like Hume's, reached essentially the same con-
clusions as Hume did—much later in a nontheological
manner—about causal statements in science and in
daily life. A second common feature was the view that
physics was concerned with efficient causes and not
with final causes. While Descartes was prepared to
admit that there were causes other than efficient
causes—for instance, that created things could be
viewed as serving God's purposes, and also that God
as causa sui was not, strictly speaking, the efficient
cause but rather the formal cause of His existence—he
insisted that in physics only efficient causes were to
be investigated. Spinoza, also imposing this restriction,
rejected outright the doctrine of final causation and
also rejected Descartes' distinction of God's formal
causation of Himself and efficient causation of the

In Principia philosophiae (1644), Descartes, laying
down the rule that efficient and not final causes were
to be investigated (Part I, Princ. xxviii), was concerned
to banish several kinds of explanations in physics.
Although he thought that ends or purposes could be
attributed to creatures having a soul or mind and also
to God, “God's purposes... seem to be hidden in the
abyss of His inscrutable wisdom”; and, while some
parts of nature served our purposes, to say that they
were created for this reason would have been con-
jecture and not an expression of genuine knowledge.
Proposing this rule, Descartes also had in mind certain
kinds of statements and explanations that he found in
scholastic manuals, for example, that heavy bodies
sought the center of the earth. To say that heavy bodies
sought the center of the earth was to impute to these
bodies characteristics that, according to Descartes,
could only be ascribed to beings having a soul or mind.

In letters to the Princess Elizabeth (May 21 and June
28, 1643, in Correspondance, ed. Adam and Milhaud,
V, 289-92, 322-25), he distinguished four primitive
notions under which all of our ideas could be subsumed
and pointed out what he took to be misapplications


of certain ideas, or category mistakes, in scholastic
physics. In addition to (1) ideas that were applicable
to any conceivable entity—e.g., being, number, and
duration—there were (2) ideas that could be applied
only to bodies, namely, ideas of their extension, shape,
and motion; (3) ideas under the category of thought
that pertained only to souls or minds; and (4) ideas
applicable to the union of soul and body in a human
being, including an idea of the “force that the soul
has of moving the body.” When, according to Des-
cartes, the scholastics explained the behavior of heavy
bodies by asserting that their weight made them seek
the center of the earth, they misapplied ideas of the
third and fourth categories. They conceived of weight
as an unextended entity that was supposed to be in
some way attached to an extended body yet could also
be removed if the body ceased to be heavy. This
quasi-substantial unextended entity, or “real property,”
was, as it were, a small soul; and the animism of the
scholastics consisted in part in their imputing to in-
animate objects entities with characteristics of the third
category that could only be applied to beings having
a soul or mind. They also thought of the weight of
a body as exerting force and acting on the body; and,
so doing, they misapplied an idea of the fourth cate-
gory—namely, of the force that the soul has of moving
the body—to entities that were to be described exclu-
sively in terms of their geometrical properties and
motion and rest.

Although Descartes took exception to what he
thought was an animistic or anthropomorphic concep-
tion of force in scholastic physics, he made use of terms
like “force” in his own physics. Stating his principle
of inertia, for instance, he maintained that “once a
body has begun to move, it has in itself a force to
continue its motion...” (letter to Mersenne, October
28, 1640). In attributing an inertial force to moving
bodies, he did not think that he was imputing to them
the kind of force experienced in voluntary action; for,
as he carefully explained, the force that he attributed
to a moving body—its “quantity of motion”—could
be defined in terms of the clearly and distinctly per-
ceived properties of motion and (ultimately) volume.

Commentators have raised questions about Des-
cartes' attempt to explain what he meant by terms like
“force” and “tendency to move” by reference to mo-
tion and rest and the geometrical properties of bodies.
In his physics, he distinguished the motion that a body
tended to have from its actual motion; and, making
this distinction, it seems that he could not consistently
identify force, or quantity of motion, with the product
of mass or volume and actual motion. In the last year
of Descartes' life, Henry More raised an objection of
this kind: he was unable to reconcile Descartes' con
tention that motion, like shape, was a mode or state
of a body and his assertion that motion could be trans-
ferred or communicated from one body to another. In
his reply, Descartes found it necessary to make a dis-
tinction. Motion, in the sense of translation from place
to place, was, like shape, a mode or state of a body.
But this was to be distinguished from a body's moving
force (vis movens), which he explained “can be God's,
conserving the same amount of motion in matter as
he placed in it from the first moment of creation; or
also that of a created substance, such as our mind; or
that of any other thing to which God gave the force
of moving a body” (August, 1649, Correspondance,
VIII, 264). This explanation left the question open as
to whether God had bestowed this force on bodies
themselves. But Descartes added that he had not dis-
cussed the matter in his published writings for fear of
giving the impression that, on his view, God was the
soul of the world; and he implied, though he did not
clearly assert, that the moving force of bodies was not
to be attributed to bodies but to God. It seems that,
when More pointed out an ambiguity and Descartes
found it necessary to make a distinction, it was difficult
for him to say unequivocally where the force that he
had distinguished from motion was located. If he at-
tributed it to bodies, he would have been imputing
to them a property admittedly other than motion and
extension, and his own view would have been subject
to the kind of objection that he had raised regarding
the forms and qualities of the scholastics. Yet he needed
the notion of a body's force, or quantity of motion,
to explain its behavior. His last words on the subject
expressed an inclination to consign this putative prop-
erty of bodies of God.

In his published writings, Descartes had claimed that
God was the primary and universal cause of motion
(Principia, Part II, Princ. xxxvi), and he had explained
that, but for God's imparting motion to matter in the
beginning and continuing to impart motion by his
“ordinary concourse,” matter would have been a
homogeneous substance undifferentiated by the motion
of its parts. He was clearly on record that the divine
concourse, or God as causa secundum esse, was a
necessary condition of the motion of bodies. Accepting
this view, the occasionalists employed a variety of
arguments to show that God was also the sufficient
condition of a body's moving, and that a body of itself
lacked the power to continue its motion or to commu-
nicate it to other bodies. In Méditations chrétiennes
et métaphysiques
(1683), Malebranche argued that, in
conserving a body from moment to moment, God must
continue to will either that it exist in the same place
or that it exist in different places. If He willed that
it exist in the same place, the body was necessarily


at rest. God's will was inviolable, and nothing could
make a body move that He had willed to be at rest.
In similar fashion, if God willed that a body exist
successively in different places, nothing could keep it
from moving to those places. Since the motion or rest
of bodies was determined necessarily by the will of
God, Malebranche concluded that bodies could not
move themselves or other bodies and that the moving
force of bodies was the will of God.

To Malebranche, the inefficacy of bodies—and in-
deed of any “second cause”—seemed to be a direct
consequence of Descartes' doctrine of divine con-
servation. But he also supported his conclusion in other
ways. In Éclaircissement XV, appended to his De la
recherche de la vérité
... (1674-75), he maintained that
we could not form a clear idea of the putative force
in bodies nor indeed of the force allegedly exercised
in human volitions. He held that, when we consulted
our clear and distinct idea of the essence of a body,
we discovered that it was necessarily extended, divisi-
ble, and movable; but, in consulting this idea, we could
not discover the force that it was supposed to have
to move itself or other bodies. If such a force could
be discovered from an investigation of an idea of a
body, it would have been possible to determine a priori
and without recourse to experience how it would move.
It would have been possible, for example, to determine
a priori and without recourse to experience that, when
a billiard ball in motion came in contact with another
ball that was at rest, the second ball would move off
in a certain direction. There was, however, no neces-
sary connection between the motion of the first ball
and that of the second, and the behavior of the second
ball could not be determined a priori. It was only from
experience that we learned that the second ball would
move in a certain way. Since our experience of regu-
larities in the behavior of moving bodies was abundant,
the mind moved with great facility from the thought
of the first ball coming in contact with the second to
the thought of the second ball moving; and, as a result,
we tended to think that there was a necessary connec-
tion and that one event was the true cause of the other.
This, however, was a mistake. The two events were
distinct and not necessarily connected; and, for Male-
branche, the only necessary connection to be discov-
ered between distinct events was between the volition
of an omnipotent being and its execution.

In similar fashion, Malebranche argued against the
efficacy of human volitions. To show that we lacked
a clear idea of the mind's alleged power to move the
body, he maintained that there was only a contingent
connection between a volition and movement of the
part of the body that was willed. Moreover, the effort
that we sometimes experienced in attempting to move
a part of our bodies did not provide us with an idea
of efficacy or necessary connection, for effort of this
sort was not always successful. Like Hume, he also
appealed to our ignorance of the cerebral mechanics
required to move parts of the body and to our failure
to understand how a volition could bring about motion
of physical particles in the brain.

Although Melebranche proclaimed that God does
everything (Dieu fait tout), he did not deny outright
or unqualifiedly that bodies or minds were causes.
Following the precedent of Géraud de Cordemoy in
Le Discernement du corps et de l'âme (1666), he distin-
guished the real or true cause, on the one hand, and
the occasional or particular or natural cause, on the
other. When one billiard ball came in contact with
a second and the second moved, the impact of the first
ball could be called the occasional cause of the second
ball's moving. To say that it was the cause in this sense
was simply to say that the second ball moved on the
occasion of contact with the first ball and that events
of the one kind regularly followed events of the other
kind. About the analysis of causal statements of this
sort, Malebranche and Hume were of the same mind.
Hume was aware of this similarity, but he chose to
stress the difference between his view and that of the
occasionalists. The occasionalists distinguished two
kinds of causes; whereas, according to Hume, “all
causes are of the same kind...,” and there was “no
foundation for that distinction... betwixt cause and
occasion.” It is fair to say that, in rejecting this distinc-
tion and claiming that all causes were of the same kind,
Hume expressed what was truly original in his view
about causation.

Benedict Spinoza, rejecting the Cartesian distinction
of God and nature, also took exception to certain
Cartesian views about causation. While Descartes ban-
ished final causes from physics on the ground that they
were unknowable, Spinoza, in the famous Appendix
to Part I of his Ethica (published posthumously, 1678),
maintained that the notion of final causation in nature
or in God was rationally indefensible. To suppose, he
argued, that God acted for certain ends entailed that
He sought something of which He was in need, and
this consequence was incompatible with the divine
perfection. Earlier, in Part I of the Ethica, he had also
argued that the supposition of God acting for an end
could not be reconciled with divine omnipotence or,
properly interpreted, divine freedom. Rejecting the
view that things in nature were created to serve our
ends, he maintained that everything followed of neces-
sity from the divine nature; and, contrary to Descartes,
he claimed that God was cause of Himself in the same
sense in which He was cause of all other things, namely,
as efficient cause. It has been noted, however, that


Spinoza's conception of an efficient cause was unlike
Descartes' and also Hume's. While agreeing with
Descartes that an efficient cause need not be tempo-
rally prior to its effect, he conceived of God as causa
that is, as not producing anything outside
of, or distinct from, Himself; and, though he antici-
pated Hume's view that all causes were of the same
kind, Spinoza's God and also the fixed and eternal
things causing individual mutable things were not
temporally prior to, nor in reality distinct from, their