The Cartesian system
Metaphysically
and epistemologically, Cartesianism is a species of
rationalism
, because Cartesians hold that knowledge?indeed, certain knowledge?can be derived through reason from
innate ideas
. It is thus opposed to the tradition of
empiricism
, which originated with
Aristotle
(384?322
bce
) and according to which all knowledge is based on sense experience and is therefore (because sense experience is fallible) only probable. In practice, however, Cartesians developed probabilistic scientific theories from observation and experiment, as did empiricists. Cartesians were forced to satisfy themselves with uncertainty in
science
because they believed that
God
is
omnipotent
and that his will is entirely free; from this it follows that God could, if he so wished, make any apparent
truth
a falsehood and any apparent falsehood?even a logical contradiction?a truth. The human intellect, by contrast, is finite; thus, humans can be certain only of what God reveals and of the fact that they and God exist. Descartes argues that one has certain knowledge of one’s own
existence
because one cannot think without knowing that one exists; this insight is expressed as “
Cogito, ergo sum
” (Latin: “I think, therefore I am”) in his
Discourse on Method
(1637) and as “I think, I am” in his
Meditations
(1641). In the
Meditations
, Descartes also argues that because we are finite, we cannot generate an idea of infinity, yet we have an idea of an
infinite
God, and thus God must exist to cause us to have that idea. He also says that although we have no direct acquaintance with the material world, not even with our own bodies, but only with ideas that represent the material world, we cannot know the material world directly. We know it exists only because God is not a deceiver.
Cartesians adopted an ontological
dualism
of two finite substances,
mind
(spirit or soul) and
matter
. The essence of mind is self-conscious thinking; the essence of matter is extension in three dimensions. God is a third, infinite
substance
, whose essence is necessary existence. God unites minds with bodies to create a fourth,
compound
substance, human beings. Humans obtain general knowledge by contemplating innate
ideas
of mind, matter, and God. For knowledge of particular events in the world, however, humans depend on bodily motions that are transmitted from sense organs through nerves to the brain to cause sensible ideas?i.e., sensations?in the mind. Thus, for Cartesians, knowledge of the material world is indirect.
This dualism of mind and matter gives rise to serious problems concerning causal interaction and
knowledge
. Given that mind and matter are so radically different, how can the body cause the mind to have sensible ideas? Likewise, how can the mind cause the body to move? How can the mind know the material world by way of sensible ideas, which are mental? In other words, how can ideas represent the properties of material objects, given that mind and matter are essentially distinct? Various lines of Cartesian
philosophy
developed from different answers to these questions.
Descartes’s philosophy is rooted in his
mathematics
. He invented
analytic
geometry?a method of solving geometric problems algebraically and algebraic problems geometrically?which is the foundation of the infinitesimal calculus developed by
Sir Isaac Newton
(1642?1727) and
Gottfried Wilhelm Leibniz
(1646?1716). The method discussed in his
Discourse on Method
is basically an extension of analytic mathematical method, which he applies to all branches of science.
The first Cartesians were Dutch and French physicists and physiologists who attempted to explain physical and biological phenomena solely in mechanistic terms?i.e., solely in terms of matter and its motion and especially without appeal to Aristotelian notions such as
form
and final cause. Descartes’s first
disciple
in the Netherlands,
Henricus Regius
(1598?1679), taught Cartesian
physics
at the University of Utrecht?though, to Descartes’s
chagrin
, he dismissed Descartes’s
metaphysics
as irrelevant to science. Another disciple, the French theologian and philosopher
Nicolas Malebranche
(1638?1715), believed with Descartes that animals are merely machines and thus incapable of thought or feeling; he is said to have kicked a pregnant dog and then to have
chastised
critics such as
Jean de La Fontaine
(1621?95), the French writer of animal fables, for expending their emotions over such inconsiderable creatures rather than concerning themselves with human misery. In Paris, the lectures of
Pierre-Sylvain Regis
(1632?1707) on Cartesian physics?which he accompanied with spectacular demonstrations of physical phenomena such as optical illusions?created such a sensation that
Louis XIV
forbade them. Because Cartesianism challenged the traditional Aristotelian science, which was supported by the
Roman Catholic Church
, and because the church also stood behind the so-called “divine right” of kings to rule, the king feared that any
criticism
of traditional authority might give rise to revolution. (Later, in the 18th century, Descartes’s emphasis on the ability of each individual to think for himself lent support to the cause of republicanism.)
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Advancements in mechanical arts and crafts provided the practical foundation of Cartesian
mechanism
. In the 17th century, mechanical
inventions
such as statues that walked and talked by application of levers and pullies and organs that played by waterpower were well known. The mathematician
Blaise Pascal
(1623?62) invented a calculating machine based on principles worked out by clock makers and inventors of spinning and knitting machines, such as the Englishman
William Lee
. The first inventors directly inspired by Descartes were the French craftsman Jean Ferrier, who attempted to make hyperbolic lenses according to Descartes’s designs, and Etienne de Villebressieu, who with Descartes’s collaboration developed an improved water pump.
Mechanism was promoted by one of Descartes’s contemporaries, the mathematician and philosopher
Marin Mersenne
(1588?1648).
Pierre Gassendi
(1592?1655) attempted to derive it theoretically from the
atomism
of the ancient Greek philosopher
Epicurus
(341?270
bce
), who held that reality is ultimately
constituted
of “atoms” in motion in the “void.” Motion was first studied scientifically by the Italian mathematician and astronomer
Galileo
(1564?1642).
According to Descartes, the material
universe
consists of an indefinitely large plenum of infinitely divisible matter, which is separated into the subtle matter of space and the denser matter of bodies by a determinate quantity of
motion
that is imparted and conserved by God. Bodies swirl like leaves in a whirlwind in vortices as great as that in which the planets sweep around the Sun and as small as that of tiny spinning globes of light. All bodily joinings and separations are mechanical, resulting from the
collisions
of other moving bodies. Because the amount of motion is conserved according to the laws of nature, the Cartesian material world exhibits a kind of
determinism
. After the initial impulse, the world evolves lawfully. If the speeds and positions of all the whirling portions of matter in the universe at any one moment could be completely described, then a complete description of their speeds and positions at any later time could be deduced through calculations based on the laws of motion. Of course, only God has the infinite intellect required for performing these calculations.
Although God is the primary cause of the existence of the material universe and of the laws of nature, all physical events?all movements and interactions of bodies?result from secondary causes?that is, from bodies colliding with each other. God stands merely for the uniformity and consistency of the laws of nature. This led
Blaise Pascal
to complain that the only purpose God serves in Descartes’s system is to initiate motion in the material world and to guarantee its
conservation
and the uniformity of nature.
Cartesianism was criticized in
England
by the Platonist philosopher
Henry More
(1614?87) and was popularized by
Antoine Le Grand (1629?99), a French Franciscan, who wrote an exposition of the Cartesians’ ingenious account of
light
and
colour
. According to popular versions of this account, light consists of tiny spinning globes of highly elastic subtle matter that fly through the air in straight lines and bounce like balls at angles consistent with the optical laws of reflection and refraction. Different colours are caused by the globes’ different speeds and spins, which themselves are determined by the texture of the surfaces on which the globes are reflected, refracted, or
transmitted
. The
spectrum
of colours observed when light passes through a triangular prism is explained by the fact that the globes pass more slowly through thicker parts of the prism than they do through thinner ones. The same spectrum of colours occurs when light passes through thicker and thinner parts of raindrops, giving rise to rainbows. Although
Newton
and
Leibniz
later showed that the simple mechanistic principles underlying these accounts were incapable of explaining the forces of gravitation and
chemical bonding
, it is noteworthy that the Cartesian theory of light is similar in principle to the contemporary view, according to which the different colours are produced by light at different wavelengths.
By the end of the 17th century, most of Cartesian physics had been superseded by Newtonian
mathematical physics
. Cartesians admitted that Descartes’s laws of motion were wrong and that his principle of the conservation of motion should be abandoned in favour of Newton’s principles of the
conservation of energy
, or
vis viva
(Latin: “living force”), and linear momentum. Although the
Treatise
(1671) of
Jacques Rohault, a leading expositor of Cartesian physics, was translated into English in 1723 by Newton’s disciple
Samuel Clarke
(1675?1729) and Clarke’s brother, their corrections and
annotations
turned the work into an exposition of Newtonian physics. Nevertheless, this progress would have pleased Descartes, who said that the advancement of scientific knowledge would take centuries of work.