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|Summary=[[File:Descartes.jpg|right|400px]]'''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)]] Aristotelians had maintained that intuition schooled by experience was the route to knowledge. Descartes, 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)]] put forward a rationalist scientific methodology 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)]]|Historical Context=[[File:La Fleche College.jpg|right|400px]]The [[Scientific Mosaic|scientific mosaic]]of the late sixteenth and early seventeenth centuries was based primarily on the works of Aristotle and some later Hellenistic natural philosophers, reconciled in various ways with Christian theology by scholars in the High Middle Ages. This '''Aristotelian-scholastic mosaic''' included Christian theology, humoral physiology, astrology, Ptolemaic astronomy, and Christian (Catholic, in many but not all communities contemporaneous with Descartes) theology.[[CiteRef::Haldane (1905)]] Descartes was well educated in this tradition through his attendance at the prestigious Jesuit La Fleche College between the ages of ten and eighteen. He studied a traditional scholastic curriculum of logic, grammar, philosophy, mathematics, and theology. Natural philosophy was taught from the works of Aristotle as interpreted by Christian scholars. Descartes also received an education in mathematics that was unusual for the Aristotelian tradition, and excelled at math. [[CiteRef::Gaukroger (1995)|pp. 38-61]][[CiteRef::Rodis-Lewis (1992)]][[CiteRef::Garber (1992)]]
Descartes’ major writings came in a time of social and intellectual upheaval in Europe. He was a participant in the Thirty Years War before writing his major works and traveled extensively around Europe at a time when the continent was embroiled in both reformation and counter-reformation, both of which were a wellspring of new thought in theology and philosophy. The community of the time was engaged with major challenges to the Aristotelian-scholastic tradition. These came from a variety of sources, including various varieties of Platonism, Hermeticism, the Chemical Philosophy of Paracelsus, among other movements.[[CiteRef::Garber (1992)]] There were new developments in optics, astronomy, and physiology.[[CiteRef::Cottingham (1992)]] Aristotle's earth-centered cosmology had been challenged by the work of Nicolaus Copernicus(1473-1543), Johannes Kepler(1571-1630), and Galileo Galilei(1564-1642), which Descartes was familiar with.[[CiteRef::Hatfield (2016)]][[CiteRef::Rodis-Lewis (1992)]][[CiteRef::Ariew (1986)]]
===Cartesian Methodology===
Under the Aristotelian scholastic [[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)]]
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Descartes scientific theories about the natural world were grounded in a metaphysical foundation, in turn deduced by the application of his rationalist methodology. He wrote that "the whole of philosophy is like a tree. The roots are metaphysics, the trunk is physics, and the branches emerging from the trunk are all the other sciences, which may be reduced to three principle ones, namely medicine, mechanics, and morals".[[CiteRef::Clarke (1992)|p. 271]] One ought to construct a metaphysics first, based on criteria independent of observation, and subsequently consider physical theories consistent with the metaphysical foundation. His natural philosophy was in stark contrast to the accepted Aristotelianism. In Aristotelian natural philosophy all objects were a compound of form and matter, a concept called hylomorphism. Form gives material bodies their distinctive properties, and makes them different from one another. It explains why fire rises and stones fall. Matter is what all material bodies share in common. All things have teleological goals or purposes. [[CiteRef::Shields (2016)]]
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 its Aristotelian form, determines its properties. Since corpuscles are too small to be directly observed, their size and shape must 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. [[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 (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 around the central sun. Earth, as a moving planet, is the center of its own smaller vortex. 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, comprised 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 Aristotle's intellectual soul. Animals, according to Descartes, are complex automata composed of physical substance only and cannot be said to think 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)]]
The overthrow of the Aristotelian tradition, even in places where Cartesianism was rejected and the community maintained Aristotelianism, forced the academic community in Europe to reconsider and defend the Aristotelian mosaic in ways that had never before been encountered. Though the dialectical approach to scholarship throughout the medieval period saw scholars constantly questioning various aspects of the Aristotelian worldview, Descartes’ wholesale rejection of huge swaths of the mosaic and its central concepts were unprecedented. Theories like hylomorphism, which had been a given in the mosaic of the sixteenth and early seventeenth centuries, and had endured through multitudes of adjustments, reconciliations, and dialectic criticism, but had never before faced complete overhaul that Descartes's mechanical natural philosophy threatened. Although Descartes' theories would eventually be supplanted by those of Newton, he made the critical first steps to replacing the Aristotelian-scholastic mosaic.[[CiteRef::Hatfield (2016)]]
|Criticism=Descartes’ ideas saw widespread criticism in his time and shortly after from all manner of sources, including Scholastic vanguards, religious authorities, and other philosophers. One common early criticism was that his new views were threatening to the Catholic and Christian faith.[[CiteRef::Jolley (1992)]] In 1663, his works were placed on the Catholic Church's ''Index of Forbidden books'', and in 1671 his conception was officially banned from schools in the Catholic world. In the early modern period, theological propositions and natural philosophical propositions were not seen as belonging to separate domains, but rather formed parts of an integrated [[Scientific Mosaic|scientific mosaic]]. Aristotelian natural philosophy had been carefully adapted to render it consistent with Catholic faith. Descartes' novel natural philosophy introduced many inconsistencies that needed to be [[The Zeroth Law|reconciled]] before his theories could be [[Theory Acceptance|accepted]]. [[CiteRef::Barseghyan (2015)|p. 190-196]]
One specific theological criticism of Cartesian natural philosophy had to do with the Catholic sacrament of the holy Eucharist, in which bread and wine are said to be transformed into the body and blood of Christ. The dogma of the ''Real Presence'' maintained that in this sacrament, Christ is really (as opposed to metaphorically or symbolically) present in the bread and wine. Centuries earlier, the Catholic saint, Thomas Aquinas(1225-1274)had posited an Aristotelian explanation for the ''Real Presence'' which had become the accepted Catholic ''doctrine of transubstantiation''. The doctrine held that in the Eucharist, the Aristotelian substance of the bread and wine were replaced by the body and blood of Christ, while their forms (that of bread and wine) remaining unchanged. In Descartes' corpuscularism, bread and wine differed from flesh and blood because they had a different arrangement of corpuscles. There is no obvious way that one could appear as the other. The Anglican church, the state church of England following its break with the Catholic papacy in the 1530's did not accept the ''doctrine of transubstantiation''. Thus Cartesianism did not face objections based on it, and became accepted at Cambridge University by 1680. Catholic Paris didn't accept it until 1700. Many solutions reconciling Cartesianism and the ''Real Presence'' had, by then, been proposed. Barseghyan speculates that the one accepted by the Parisian community was that proposed by Antoine Arnauld(1612-1694)in 1671, in which Christ's presence in the Eucharist was due to a miracle beyond human comprehension, and outside the ordinary course of nature described by Cartesianism.[[CiteRef::Barseghyan (2015)|p. 194]]
Critics in Decartes' era were also concerned that corpuscular explanation involved hypothetical unobservable entities, and the supposition that this invisibly small world could be understood by analogy with larger objects. Descartes countered that "there is nothing more in keeping with reason that we judge about those things that we do not perceive, because of their small size, by comparison and contrast with those that we see" [[CiteRef::Clarke (1992)|p. 267]] He felt that a plausible model, though potentially incorrect due to the unobservability of its fundamental parts, was better than none at all. The role of unobservably small entities in the physical sciences was to remain a matter of prolonged debate. In modern science, it is an accepted and central practice.
Another notable objection came from Princess Elizabeth of Bohemia (1618-1680), who questioned Descartes’ theory of substance in a letter dated the tenth of May, 1643. In it, she asks “Given that the soul of a human being is only a thinking substance, how can it affect the bodily spirits, in order to bring about voluntary actions?”[[CiteRef::Descartes (2009)]] Descartes would never gives a satisfactory answer to this central question over the course of the correspondence. In 1747, in his ''L'Homme machine''(''Machine Man'') Julien Offray de La Mettrie (1709-1751) raised another simple, but devastating objection to Descartes' supposition that human reason must be due to an immaterial mental substance. He noted that reason can become impaired by material causes such as drunkenness and fever. [[CiteRef::De La Mettrie (1996)]]. In the nineteenth century, Princess Elizabeth's objection became far more poignant with the formulation of the law of the conservation of energy, which implied causal closure of the physical world to influence by a mental substance. In the early twentieth century, the mathematician Alan Turing (1912-1954) showed that it was possible to construct a general purpose machine capable of performing any possible mathematical computation, thereby demonstrating that a general purpose machine was possible and refuting Descartes' core argument against a mechanical understanding of human reason. By the end of the twentieth century, the relevant scientific communities of neuroscience and cognitive science had rejected the idea of a mental substance and sought a mechanistic physical explanation for the mind [[CiteRef::Bechtel (2008)]], though there was still no agreement as to whether consciousness could be explained in this fashion. [[CiteRef::Chalmers (1996)]]
