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| Jacques Rohault |
Jacques Rohault was a mechanistic Cartesian experimental physicist. He was born in Amiens, France, and earned his MA in Paris in 1641. There, he became Claude Clerselier's Cartesian disciple and son-in-law. He was Pierre-Sylvain Régis's teacher and converted him to Cartesianism.
In the 1650s Rohault was a private tutor in Paris, and his "Cartesian Wednesday" evening lectures, complete with laboratory table demonstrations, were attended by many members of the noble class, women as well as men, and did a great deal toward popularizing Cartesianism.
His Traite de physique (Paris, 1671) was a standard text for nearly fifty years. John Clarke and Samuel Clarke, rather than writing a Newtonian physics, translated Rohault's work into Latin (1697) and English (1723) and added Newtonian footnotes to correct Rohault's Cartesian mistakes.
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| Samuel Clarke |
The Traite contains descriptions of explanations and experiments in support of Cartesian mechanistic physics. Like René Descartes, Rohault holds that these explanations are only probable because absolute certainty is unattainable by humans.
Also in Paris in 1671 Rohault published his Entretiens sur la philosophie, in which he defends the thesis that Cartesian principles and Christian doctrines do not conflict because each pertains to a separate and distinct realm of truth and knowledge. The book was popular, but Rohault's position was generally viewed as heretical by the Catholic Church.
Rohault opposes Nicolas Malebranche's occasionalism and presents his own mechanistic Cartesianism based on eight axioms he takes to be self-evident:
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| Entretiens sur la philosophie |
- Nothing (that which has no existence) has no properties
- Something cannot possibly be made of nothing, that is, nothing cannot become something
- No thing or substance can be annihilated, that is, something cannot be reduced to nothing
- Every effect presupposes some cause
- If one does not cause an effect, that effect necessarily depends on some othercause
- Everything endeavors to continue in the state in which it is (an early Cartesian rendering of a principle of inertia)
- Every alteration is made by some external cause, that is, in opposition to Aristotle, no material thing can alter itself through an inner power, force, or form
- Every alteration is proportional to the force of the causal agent
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| necessary existence |
In strict Cartesian order one knows first one's own self, whose existence Rohault proves syllogistically:
- From principle (1) above, whatever has properties is something
- Thinking is a property
- Whatever thinks, therefore, exists as something because it has the property of thinking
- I think
- Therefore, I exist
Rohault states that mind and matter are completely different but that God so created the human mind or soul such that motions caused by material impressions on the sense organs and in the brain of the body with which it is united give rise in the soul to sensations and ideas.
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| material things |
Neither sensations nor ideas resemble material things, and so resemblance is not necessary for knowledge. It is simply the nature of sensations to give knowledge of the existence of material things, and the nature of some ideas is to give knowledge of the place, situation, distance, magnitude, figure, number, and motion or rest of material things.
Rohault's method in physics is to reason mathematically about experiments before conducting them. His goal is to explain the sensible effects of material things. For this only the primary material properties of size, figure, motion, and arrangement of divisible, impenetrable particles in a plenum are needed; occult qualities such as Aristotelian forms are unnecessary.
In Entretiens de philosophie (Paris, 1671), the companion volume to the Traite, Rohault explains in mechanical terms Cartesian opinions on animal machines and transubstantiation. Animal behavior, he claims, can be explained if animals are completely material; human behavior, however, requires a rational soul that is immaterial, hence indivisible, hence immortal.
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| sensory experience |
For Cartesians, the sensible qualities as they exist in material things are not seen, tasted, and so on as one sees and tastes them, but are merely the powers bodies have, determined by the size, figure, motion, and arrangement of their particles, to cause sensations in the mind. There is no further explanation of these powers beyond the fact that God made the correlations between bodily movements and one's sensory experience.
Transubstantiation, then, is the point-by-point replacement of bread and wine by Christ's flesh and blood. Therefore, the flesh and blood of Christ that occupies the places (is bound by the surfaces) formerly occupied by bread and wine causes sensations exactly like those that the bread and wine formerly caused.
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| Oeuvres posthumes de Rohault |
Consequently, real accidents or Aristotelian forms subsisting separately from Aristotelian matter as postulated in the scholastic explanation of transubstantiation are unnecessary. There are further physical explanations and assurances that Cartesian principles do not contradict Catholic doctrine in Oeuvres posthumes de Rohault (Paris, 1682).
Overall, Rohault disclaims metaphysics and says that although the substitutions are miraculous, even his mechanist explanation of transubstantiation is only a solution to a problem in physics. His work illustrates the strong empiricist stress on observation and experiment toward probable mechanistic explanations in physics so prominent in many Cartesian philosophers.
Finally, use of Rohault's Traite as a physics textbook merely with addition of Newtonian footnotes constitutes a major shift to nonmetaphysical, explanatory concerns in science.
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| Cartesian philosophers |