René Descartes

Philosophical work 

Descartes is often regarded as the first modern thinker to provide a philosophical framework for the natural sciences as these began to develop. In his Meditations on First Philosophy he attempts to arrive at a fundamental set of principles that one can know as true without any doubt. To achieve this, he employs a method called methodological skepticism: he doubts any idea that can be doubted in order to acquire a firm foundation for genuine knowledge.

Initially, Descartes arrives at only a single principle: thought exists. Thought cannot be separated from me, therefore, I exist (Meditations on First Philosophy). Most famously, this is known as cogito ergo sum, ("I think, therefore I am").

Therefore, Descartes concludes that he can be certain that he exists. But in what form? He perceives his body through the use of the senses; however, these have previously been proven unreliable. So Descartes concludes that the only undoubtable knowledge is that he is a thinking thing. Thinking is his essence as it is the only thing about him that cannot be doubted. Descartes defines "thought" (cogitatio) as "what happens in me such that I am immediately conscious of it, insofar as I am conscious of it". Thinking is thus every activity of a person of which he is immediately conscious.

To further demonstrate the limitations of the senses, Descartes proceeds with what is known as the Wax Argument. He considers a piece of wax: his senses inform him that it has certain characteristics, such as shape, texture, size, color, smell, and so forth. When he brings the wax towards a flame, these characteristics change completely. However, it seems that it is still the same thing: it is still a piece of wax, even though the data of the senses inform him that all of its characteristics are different. Therefore, in order to properly grasp the nature of the wax, he cannot use the senses: he must use his mind. Descartes concludes:

"Thus what I thought I had seen with my eyes, I actually grasped solely with the faculty of judgment, which is in my mind."

In this manner, Descartes proceeds to construct a system of knowledge, discarding perception as unreliable and instead admitting only deduction as a method. Halfway through the Meditations, he offers an ontological proof of a benevolent God (through both the ontological argument and trademark argument). Because God is benevolent, he can have some faith in the account of reality his senses provide him, for God has provided him with a working mind and sensory system and does not desire to deceive him; however, this is a contentious argument, as his very notion of a benevolent God from which he developed this argument is easily subject to the same kind of doubt as his perceptions. From this supposition, however, he finally establishes the possibility of acquiring knowledge about the world based on deduction and perception. In terms of epistemology therefore, he can be said to have contributed such ideas as a rigorous conception of foundationalism and the possibility that reason is the only reliable method of attaining knowledge, as others said before him, though not as clearly as he did, and the rationalist answer to skepticism which other rationalists have elaborated on.

In Descartes's system, knowledge takes the form of ideas, and philosophical investigation is the contemplation of these ideas. This concept would influence subsequent internalist movements as Descartes's epistemology requires that a connection made by conscious awareness will distinguish knowledge from falsity. As a result of his Cartesian doubt, he sought for knowledge to be "incapable of being destroyed", in order to construct an unshakable ground from which all other knowledge can be based on. The first item of unshakable knowledge that Descartes argues for is the aforementioned cogito, or thinking thing.

Descartes also wrote a response to skepticism about the existence of the external world. He argues that sensory perceptions come to him involuntarily, and are not willed by him. They are external to his senses, and according to Descartes, this is evidence of the existence of something outside of his mind, and thus, an external world. Descartes goes on to show that the things in the external world are material by arguing that since God would not deceive him as to the ideas that are being transmitted, and that God has given him the "propensity" to believe that such ideas are caused by material things. Skeptics have responded to Descartes's proof for the external world by positing a brain in a vat thought experiment, in that Descartes's brain may be connected up to a machine which simulates all of these perceptions (though the vat itself would still be part of an external world, so this argument doesn't completely eliminate that outside the mind.)

Dualism

Descartes suggested that the body works like a machine, that it has the material properties of extension and motion, and that it follows the laws of physics. The mind (or soul), on the other hand, was described as a nonmaterial entity that lacks extension and motion, and does not follow the laws of physics. Descartes argued that only humans have minds, and that the mind interacts with the body at the pineal gland. This form of dualism proposes that the mind controls the body, but that the body can also influence the otherwise rational mind, such as when people act out of passion. Most of the previous accounts of the relationship between mind and body had been uni-directional.

Descartes suggested that the pineal gland is the seat of the soul for several reasons. First, the soul is unitary, and unlike many areas of the brain (e.g. the frontal lobes) the pineal gland is also unitary. Second, Descartes observed that the pineal gland was located near the ventricles. He believed the animal spirits of the ventricles acted through the nerves to control the body, and that the pineal gland influenced this process. Finally, Descartes (incorrectly) believed that only humans have pineal glands, just as, in his view, only humans have minds.

Cartesian dualism set the agenda for philosophical discussion of the mind-body problem for many years after Descartes' death. His pineal gland solution was ingenious, if ultimately incorrect. The question of how a nonmaterial mind can influence a material body, without invoking supernatural explanations, remains an enigma to this day.

From 1628 until his ill-fated trip to Sweden in 1649 he remained for the most part in Holland, and it was during this period that he composed a series of works that set the agenda for all later students of mind and body. The first of these works, De homine was completed in Holland about 1633, on the eve of the condemnation of Galileo. When Descartes learned of Galileo's fate at the hands of the Inquisition, he immediately suppressed his own treatise. As a result, the world's first extended essay on physiological psychology was published only well after its author's death.

In this work, Descartes proposed a mechanism for automatic reaction in response to external events. According to his proposal, external motions affect the peripheral ends of the nerve fibrils, which in turn displace the central ends. As the central ends are displaced, the pattern of interfibrillar space is rearranged and the flow of animal spirits is thereby directed into the appropriate nerves. It was Descartes' articulation of this mechanism for automatic, differentiated reaction that led to his generally being credited with the founding of reflex theory.