In previous units we have watched Descartes using a series of skeptical hypotheses to systematically question his beliefs. In doing so he moved from beliefs derived from the senses, through those of the imagination until he reached truths of the understanding. At this point he finds the cogito and other truths which are immune to skeptical doubt. These truths, self-evident, revealed by the natural light, and clear and distinct provide the base from which he will try to build up his new house of knowledge. In this unit we will follow Descartes for a ways as he builds his new house on this foundation. Unlike the original house, the new house has truths discovered by reason or understanding at the bottom, then truths of the imagination and truths of the senses.
After discovering the various truths in Meditation II, Descartes proves the existence of God in Meditation III. In doing so, he effectively eliminates the evil deceiver hypotheses. For God is perfect and hence no deceiver, but if God were to allow an evil deceiver to exist, he would thereby makes Himself a deceiver. So, there can be no evil demon. But the evil demon hypothesis was invoked to call into question those truths of mathematics which could not be doubted on the dream hypothesis. So, once the proof for the existence of God is completed, Descartes has, in effect, removed the slight doubt which kept him from believing the truths of mathematics.
Since the truths of mathematics are now certain, we know that truths which rely on any image which embodies mathematical objects such as, squares or circles, cubes, tetrahedrons, or dodecahedrons, will be clear and distinct. For the underlying truths of mathematics guarantee the truth of such claims. Insofar as material objects are considered as images governed by the truths of mathematics we can say that in this sense we know material things. This, of course, does not establish that material things exist.
Descartes give a proof from each of the faculties. We have just looked at the proof from the understanding. The proof from the imagination suggests that among the various ways in which the images of material objects which I have might have been caused, the most plausible is that they were caused by material objects. This makes the existence of material objects probable, though not certain. Finally, Descartes notes that because of the images derived from the senses of material objects, he has a strong propensity to believe that material things exist. If they do not, he finds no way to avoid the conclusion that God must be a deceiver. But God is not a deceiver, so material objects must exist.
At the end of the Meditations Descartes defeats the dream hypothesis by producing what he could not discover in Meditation I, the criterion which one might use to distinguish waking from sleeping. The criterion is that objects have certain constant causal connections in the waking world which they do not have in dreams. Typically objects, for example, do not appear and disappear when one is awake, rather they come and go away. Similarly, in waking consciousness, there is a conceitedness of the images experienced with other events in one's life, typically absent in dreams.
The very first of Descartes' skeptical hypotheses had to do with objects seen at a distance. Later he also called into question sensible qualities like sound and pain which he understands less clearly than mathematical objects. So, what is to be said about these? Descartes' reply is that particular properties like the size of the sun may well be doubtful but, God is not a deceiver and since we have a God given ability to correct the falsity in any of our beliefs, it is possible that we may acquire the truth about even these things.
Descartes comes to the conclusion in the Meditations that the mind is a separate substance from the body. Thus the mind can exist independently of the body. This is an important step towards showing that the mind or soul is immortal. In this life, however, the mind and body are closely related. Later, in other works, Descartes attempted to explain this connection. The two substances causally interact with one another. When the body causally effect the mind we get perceptions of all kinds. When the mind effects the body we get intentional actions, such as my raising my arm. It turns out it is very difficult to understand how two substances as different as the mind and body can causally effect one another. This enduring difficulty has come to be known as the mind body problem.
In Meditation VI Descartes gives two proofs to establish that there is a real distinction between minds and bodies. One of these proofs depends on Descartes having a clear and distinct idea of his essence as a thinking thing, and the essence of bodies as extended and unthinking things. The other proof starts from Descartes recognition that the mind is simple and thus indivisible, while the body is composite and so divisible. Given this difference the mind and body must be distinct.
Substance dualism means that each of us is made up of two distinct entities, a mind and a body. How do these two interact? The answer which Descartes gives is the doctrine of two way causal interaction. The first kind of causal interaction leads to perception. If the body is stimulated by lights, sounds or touches, this information is transferred to the brain and then to the mind where it is perceived. The second kind of causal interaction goes the other way. It begins with the mind and a desire and a belief that a certain bodily movement, say raising my arm should occur. The mind then transmits a command to the body, and presuming that all the bodily machinery is functioning properly, my arm goes up.
Given two way causal interaction, one may wonder where the interaction occurs. Minds, after all, are not extended, so they are not in space. Bodies are extended. In works subsequent to the Meditations Descartes claimed that the place in the brain where information was transmitted from the body to the mind and from the mind to the body was the pineal gland. Philosophers in the next generation after Descartes (and indeed up to the present day) have found the doctrine of two-way causal interaction beset with a variety of severe difficulties. This has led philosophers to try a whole range of alternative explanations to avoid two way causal interaction. All of these solutions in turn have difficulties. The problem of explaining the relation of minds and bodies has thus come to be called the mind body problem.
Descartes made significant contributions in the development of mathematics. One of the most important characteristics of this period is the algebraic treatment of all parts of mathematics. Descartes discovery of analytic geometry is a notable contribution to this trend. Analytic geometry is also important as a step towards the development of the calculus by Leibniz and Newton. In physics, Descartes developed a full blown physical theory which rivaled Newton's account for the next seventy five years. While most of Descartes' laws of physics turned out to be wrong, he was arguably the first to state the law of inertia in its modern form. Historians of science have claimed that the clear statement of the law of inertia was really the crucial event in the development of modern physics.
In the period which we are studying, the 17th and 18th centuries, Descartes had enormous influence in a variety of ways, only some of which we can catalogue here. One of the most notable of these is the way in which Descartes talk of clear and distinct or obscure and confused ideas was adopted in one form or another by virtually all the major philosophers during this era. Another is what some historians call the epistemological turn in philosophy. This is a turn away from focusing on metaphysical issues, and assuming that we know that God, minds and bodies exist, towards a central focus on issues related to our knowledge of the world. Clearly skepticism played an important role here, but Descartes did as well. Finally, Descartes is the first of the rationalist philosophers, to be followed by Spinoza and Leibniz (as well as a variety of lesser lights). All of these philosophers believed in innate ideas in one form or another, and believed the mind was more fundamental than the senses in discovering truths.
Descartes developed a strategy for defeating skepticism. This strategy involved finding truths which were self-evident and indubitable from which he could build up his entire edifice of knowledge. Such a strategy is called foundationalism because it seeks to find a certain starting point on which the rest can be built This was a strategy which persisted in philosophy well into the twentieth century. It was only in the twentieth century that philosophers came to question the possibility of finding such foundational truths and began seriously looking for a non-foundationalist way of justifying our claims to know. Similarly, Descartes attempts to establish the relation between mind and body created a whole field of inquiry exploring a variety of ways to think about the relations between minds and bodies. These represent some of Descartes most important legacies.