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|Summary='''Rene Descartes''' (1596-1650) was a French mathematician and philosopher. Descartes rejected the Aristotelian-scholastic world view accepted for most of the previous two thousand years, and laid down new foundations for knowledge.[[CiteRef::Russell (1945)|p. 524]][[CiteRef::Newman (2014)]][[CiteRef::Garber (1993)]] In mathematics he developed techniques Aristotelians had maintained that made possible analytic geometry. In natural philosophy, he intuition schooled by experience was co-framer of the sine law of light refraction, developed a theory of the rainbow, and formulated a precursor of the nebular hypothesis of the origin of the solar systemroute to knowledge.[[CiteRef::Hatfield (2016)]] Rejecting the Aristotelian world of forms, substances Descartes, and teleology, he posited a mechanical world in which matter possessed only spatial extension and interacted only by contact. In his ''Discourse on the Method of Rightly Conducting One’s Reason and of Seeking Truth in the Sciences'' (''Discourse on Method''), first published in 1637.,[[CiteRef::Descartes (2007)]] Descartes posited put forward a rationalist scientific methodology whereby in which a proposition is acceptable only if it can be clearly and distinctly perceived by the intellect beyond all reasonable doubt or follows deductively from such propositions.[[CiteRef::Newman (2014)]] This allowed him to advance a mathematical a priorist approach to scientific knowledge and inquiry.[[CiteRef::Garber (1992)]][[CiteRef::Clarke (1992)]] Rejecting the Aristotelian world of forms, substances, and teleology, he posited a mechanical world in which matter possessed only spatial extension and interacted only by contact. In mathematics he developed techniques that made possible analytic geometry. In natural philosophy, he was co-framer of the sine law of light refraction, developed a theory of the rainbow, and formulated a precursor of the nebular hypothesis of the origin of the solar system.[[CiteRef::Hatfield (2016)]]

The '''mechanical natural philosophy''' was a [[Theory Pursuit|pursued ]] radical alternative to Aristotelian cosmology, embraced by some supporters of Copernican heliocentrism.[[CiteRef::Luthy, Murdoch, and Newman (2001b)]][[CiteRef::Chalmers (2014)]][[CiteRef::Gaukroger (1995)|p. 69-73]] It rejected the Aristotelian fundamentals of form, substance, and teleology, and the idea that matter is continuous. Instead of explaining the properties of visible bodies in terms of form, it instead maintained that the world consisted of invisibly tiny particles of matter and that all the observable properties of the visible bodies were a consequence of these particles and their interactions with one another. The particles interacted mechanically, by contact, and it was often supposed that they rendered natural phenomena potentially explainable in geometrical and mathematical terms. Unlike Aristotle's physics, it was compatible with a moving planetary Earth. It can be traced to the Ancient Greek '''atomism''' of Democritus (circa 460-370 BCE) and later Epicurean philosophers. Atomism was reintroduced into European thought in the fifteenth century with the rediscovery of the Roman poet Lucretius's ''De rerum natura''. In the early seventeenth century, it was championed by Giordano Bruno (1548-1600), Nicolas Hill (1570-1610?), Sebastian Basso (1573-1625?), Francis Bacon (1561-1626), and Galileo Galilei.[[CiteRef::Garber (1992)]][[CiteRef::Klein (2012)]][[CiteRef::Gatti (2001)]][[CiteRef::Luthy, Murdoch, and Newman (2001b)]]

In terms of his methodology Descartes was largely responding to what he perceived as the dogmatism and marked lack of progress he saw in the Scholastic tradition, and his excitement with the new mechanical natural philosophy. His weariness with the largely dialectical scholastic method is what led him to develop the highly systematized epistemology and metaphysics for which he would come to be known. The Aristotelian-scholastic mosaic continued to be [[Theory Acceptance|accepted ]] throughout Descartes's life, with acceptance of his views coming later.|Major Contributions=Descartes new methodology and mechanical natural philosophy were of revolutionary importance. They became accepted at Cambridge University in England by 1680,[[CiteRef::Barseghyan (2015)|p. 211]] and in France by about 1700, displacing the Aristotelian-medieval system of theories from the [[Scientific Mosaic|scientific mosaic]]. These theories were ultimately fully displaced throughout Europe by Descartes ' theories and by the later theories of Issac Newton (1642-1726).[[CiteRef::Barseghyan (2015)|p. 167]]

Under the Aristotelian scholastic method [[Methodology|methodology]] a theory is acceptable “if it grasps the nature of a thing through intuition schooled by experience, or if it is deduced from general intuitive principles”.[[CiteRef::Barseghyan (2015)|p. 144]] Descartes became frustrated with this tradition and its dialectical approach to knowledge-seeking, which he charged with plunging him into skeptical doubts whereby he could never be sure what was true and what was not. He writes in ''Discourse on the Method'':[[CiteRef::Descartes (2007)]]

Descartes concluded that if his goal was to attain certain knowledge about the world,the accepted methodology for doing so must be rejected , and a new one would be required. Methodology held a central place in his epistemology; in fact, one of Descartes’ criticisms of Galileo was that he failed to produce a fully developed methodology to justify his discoveries, and had simply explained particular physical phenomena.[[CiteRef::Ariew (1986)]] To that end he embraced his skeptical doubts and devised a methodology based on '''methodological skepticism'''; a methodology whereby he rejects all knowledge that he cannot be certain of, accepts only those propositions which he can accept as certain, and proceed deductively from those axioms according to reason. By this method Descartes hoped to produce a kind of systematized knowledge that, he believed, could be universally acceptable. In his 'Meditations on First Philosophy', [[CiteRef::Descartes (2004)]] Descartes identified the sole indubitable proposition upon which he would build the entire rest of his philosophical system as his famous '''‘Cogito, Ergo Sum’''' (also styled ‘Dubito, Ergo Cogito, Ergo Sum’ or simply as ‘the Cogito’); “I think, therefore I am.” From this foundation Descartes deduced that he was a created thing, his requiring a creator, that creator being God, the benevolent nature of God, and the consequent reliability of his God-given senses and reason, all of which formed the broader foundation of his systematized scientific worldview.[[CiteRef::Newman (2014)]]

Although Descartes maintained some methodological aspects of the Scholastic-Aristotelian mosaic – namely the axiomatic-deductive, epistemic-foundationalist structure of investigation – one critical difference in his methodology was the shift in the method of theory choice. It jettisoned the Aristotelian expectation that a theory must be experientially based and intuitively obvious for it to be acceptable, and although his system, as it ended up, allowed for knowledge that was both experiential and intuited,[[CiteRef::Newman (2014)]] the ultimate justification for knowledge claims was human reason. Descartes was both a '''rationalist''' and an ''a priorist'', in that his epistemology and metaphysics allows for the existence of synthetic a priori propositions.

In Descartes' mechanical corpuscular natural philosophy, by contrast, there are just two kinds of substance that are entirely different from each other in kind: mental substance and physical substance. The fundamental property of '''mental substance''' was thought, and Descartes equated it with the rational soul of God and humans. The fundamental feature of '''physical substance''' was extension in space. He rejected Aristotle's distinction between form and matter, including Aristotle's four elements. [[CiteRef::Ariew (1992)]] Cartesian mechanics rejects the void posited by atomists; instead matter fills the universe as a plenum. If all matter is extended, Descartes reasoned that there can be no space without extended matter. Also unlike atomism, matter is infinitely divisible, though visible things are composed of tiny corpuscles that interact with one another by physical contact. The corpuscular composition of a material body, rather than form, determines its properties. Since corpuscles are too small to be directly observed, their size and shape is hypotheticalmust be hypothesized, though observation can allow us to infer the plausibility of our guesswork. Our senses, Descartes maintained, do not inform us of the mechanical world as it is, but provide us with sensations which are mere signs of their objective causes. Only extended matter and motion exist apart from our minds. Secondary qualities, such as colors, are created in our minds in response to mechanical stimuli. [[CiteRef::Clarke (1992)]] Descartes completed a manuscript that was to be a comprehensive expression of his mechanical natural philosophy, called ''The World''. He withdrew his plans to publish it upon learning of the condemnation of Galileo in Rome in 1633. The work never appeared during his lifetime, but two major fragments, the ''Treatise on Light'', and the ''Treatise on Man'' were published posthumously. The first dealt with physics, and the second put forward a theory of physiology, nervous system function, and the mind/brain relationship. [[CiteRef::Garber (1992)]][[CiteRef::Descartes (1664/2003)]]

In Descartes cosmology, the universe is essentially mechanical in character. Copernican heliocentrism is accepted, and planetary motion is explained in terms of a swirling '''vortex''' of material particles. Earth, as a moving planet, is the center of its own smaller vortex. This The particles of the vortex push larger bodies towards its center and this explains gravity without supposing, as did Aristotle, that the sphere of earth was at rest in its natural place; the center of the universe. It also made it reasonable to suppose that other planets had their own attractive vorticies, and were thus other worlds. [[CiteRef::Garber (1992)]]

Descartes also challenged Aristotelian physiology. Aristotle's theory of physiology posited three souls or vital principles, the nutritive soul, responsible for nutrition and reproduction, and comprising the entirety of the soul in plants, the sensitive soul, responsible for perception, locomotion, imagination, and desire, was added to the sensitive soul in animals. A third component, the intellectual soul was found uniquely in human beings. [[CiteRef::Shields (2016b)]][[CiteRef::Van der Eijk (2000)]] Descartes rejected the nutritive and sensitive souls, supposing their functions were instead performed by corpuscular mechanisms, the nature of which he outlined in his ''Treatise on Man''. [[CiteRef::Descartes (2007)]] Descartes ' mental substance served roughly the same role as Aristotles' intellectual soul. Animals, according to Descartes, are complex automata composed of physical substance only and cannot be said to think or feel in the way that human beings or God can; these properties being made possible by mental substance . [[CiteRef::Des Chene (2001)]][[CiteRef::Clarke (1992)]][[CiteRef::Descartes (2007)]]. Descartes posited a mental substance for theological, metaphysical, and scientific reasons. He supposed that thought could not be mechanized, since all the machines known to him were specialized to perform one particular function, but human reason was a general purpose instrument.[[CiteRef::Hatfield (1992)]][[CiteRef::Cottingham (1992)]]