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|Brief=a French natural philosopher and scientist; he who is today considered one of the most influential figures in the modern philosophy

|Historical Context=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, . This was reconciled in various ways with Christian theology by scholars in the High Middle Ages. This '''Aristotelian-scholastic mosaic''' included Christian theologyPtolemaic astronomy, astrology, 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 Before writing his major works, he was a participant in the Thirty Years War before writing his major works and traveled . He travelled extensively around Europe at a time when the continent was embroiled in both reformation and counter-reformation, both of which were both 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 varied sources, including various varieties of Platonism, Hermeticism, and 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)]]

The After leaving La Fleche, in 1618,Descartes became involved in a collaboration with the Dutch Calvinist natural philosopher Isaac Beeckman (1588-1687), who valued him for his mathematical skills. They worked together on several mathematical problems in natural philosophy. [[CiteRef::Gaukroger (1995)|p. 68]] Beeckman was a supporter of the '''mechanical natural philosophy''' . This 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 maintained that the world consisted of invisibly tiny particles of matter and that all the observable properties of visible bodies were a consequence of these particles and their interactions with one another. Particles interacted The particles were held to interact mechanically, by contact, and it was often supposed that this rendered natural phenomena potentially explainable in geometrical and mathematical terms. Unlike Aristotle's physics, the mechanical philosophy 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''. Part of its appeal lay in the fact that unlike Aristotle's physics, the mechanical philosophy was compatible with a moving planetary Earth. 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)]]

After leaving La Fleche, in 1618,Descartes became involved in a collaboration with the Dutch Calvinist natural philosopher Isaac and Beeckman (1588-1687), who valued him for his mathematical skills. They worked together on several mathematical problems in natural philosophy. Beeckman was a corpuscularist. A derivative of atomism, '''corpuscularism''' rejected indivisible atoms and void spaces but nonetheless accounted for the properties of objects in terms of invisibly tiny particles. [[CiteRef::Gaukroger (1995)|p. 68]] Beeckman is almost certainly the first person in Europe to attempt to explain macro-geometrical regularities in terms of micro-mechanical models. [[CiteRef::Gaukroger (1995)|p. 70]] For the most part, applying mathematics to physical problems was not part of the Aristotelian tradition. Descartes adopted Beeckman's mathematical corpuscularism and became part of a community of corpuscularist thinkers which besides Beeckman and Descartes included Marin Mersenne (1588-1648), Thomas Hobbes (1588-1679), Pierre Gassendi (1592-1655), Sir Kenelm Digby (1603-1665), and Walter Charleston (1620-1707). They all knew each other and reacted to each other's work.[[CiteRef::Osler (2001)]] The decade after Descartes met Beeckman was the most philosophically productive of his life. [[CiteRef::Garber (1992)]]

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 requiredformulated. 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)]] Rather than experience and intuition, Descartes' '''methodological skepticism''' was based on reason and on the capacity to doubt. The harder a proposition was to doubt, the greater its certainty. This was an epistemological innovation.

His strategy was, first, to reject all knowledge that he cannot be certain of, and accept only those propositions of which he is certain. He would deduce other knowledge from such axioms using reason. By this method Descartes hoped to produce a kind of systematized knowledge that could be universally accepted. In his ''Meditations on First Philosophy'', [[CiteRef::Descartes (2004)]] Descartes identified the sole indubitable proposition upon which he would build his entire philosophical system: he was certain of his own existence as a thinking being, or in latinLatin, '''''‘Cogito, Ergo Sum’''''' (also styled ''‘Dubito, Ergo Cogito, Ergo Sum’'' or simply as ''‘the Cogito’''); “I think, therefore I am.” From the foundation of his own existence, Descartes deduced that he must be a created being, that this requires a creator, that creator being God, the benevolent nature of God, and the consequent reliability of his senses and of the God-given ability of his reason to form clear and distinct ideas. It was therefore possible to use his senses and reason to gain knowledge of an external world. This reasoning formed the foundation of his systematized scientific worldview.[[CiteRef::Newman (2014)]]

Descartes maintained some aspects of the Scholastic-Aristotelian methodology – namely an axiomatic-deductive, epistemic-foundationalist structure of investigation. But the critical difference in his methodology was the shift in the method of theory choice. It He 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 and the absence of doubt. Descartes was both a '''rationalist''' and an '''a priorist''', in that his epistemology and metaphysics allows for the existence of synthetic a priori propositions. Although an argument for God's existence was at the foundation of his system, Descartes' rationalism was nonetheless a formidable challenge to the accepted theological methodology which had been comprehensively expressed by the Catholic saint, Thomas Aquinas (1225-1274) more than three hundred years earlier. Aquinas saw human reason as limited, and always to be exercised in the context of, and subject to the authority of, the divine revelation of the Bible. Descartes, by contrast, sought to develop his epistemology and theology on the basis of human reason alone. [[CiteRef:: Hyman (2007)]]

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]]