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At the outset of the Third Meditation, Descartes tried to use this first truth as the paradigm for his general account of the possibilities for achieving human knowledge. In the cogito, awareness of myself, of thinking, and of existence are somehow combined in such a way as to result in an intuitive grasp of a truth that cannot be doubted. Perhaps we can find in other cases the same grounds for indubitable truth. But what is it?
The answer lies in Descartes's theory of ideas. Considered formally, as the content of my thinking activity, the ideas involved in the cogito are unusually clear and distinct. (Med. III) But ideas may also be considered objectively, as the mental representatives of things that really exist. According to a representative realist like Descartes, then, the connections among our ideas yield truth only when they correspond to the way the world really is. But it is not obvious that our clear and distinct ideas do correspond to the reality of things, since we suppose that there may be an omnipotent deceiver.
In some measure, the reliability of our ideas may depend on the source from which they are derived.
Descartes held that there are only three possibilities: all of our ideas are either adventitious (entering the mind from the outside world) or
factitious (manufactured by the mind itself) or
innate (inscribed on the mind by god).
But I don't yet know that there is an outside world, and I can imagine almost anything, so everything depends on whether god exists and deceives me.
The next step in the pursuit of knowledge, then, is to prove that god does indeed exist. Descartes's starting point for such a proof is the principle that the cause of any idea must have at least as much reality as the content of the idea itself. But since my idea of god has an absolutely unlimited content, the cause of this idea must itself be infinite, and only the truly existing god is that. In other words, my idea of god cannot be either adventitious or factitious (since I could neither experience god directly nor discover the concept of perfection in myself), so it must be innately provided by god. Therefore, god exists. (Med. III)
As a backup to this argument, Descartes offered a traditional version of the cosmological argument for god's existence. From the cogito I know that I exist, and since I am not perfect in every way, I cannot have caused myself. So something else must have caused my existence, and no matter what that something is (my parents?), we could ask what caused it to exist. The chain of causes must end eventually, and that will be with the ultimate, perfect, self-caused being, or god.
As Antoine Arnauld pointed out in an Objection published along with the Meditations themselves, there is a problem with this reasoning. Since Descartes will use the existence (and veracity) of god to prove the reliability of clear and distinct ideas in Meditation Four, his use of clear and distinct ideas to prove the existence of god in Meditation Three is an example of circular reasoning. Descartes replied that his argument is not circular because intuitive reasoningin the proof of god as in the cogitorequires no further support in the moment of its conception. We must rely on a non-deceiving god only as the guarantor of veridical memory, when a demonstrative argument involves too many steps to be held in the mind at once. But this response is not entirely convincing.
The problem is a significant one, since the proof of god's existence is not only the first attempt to establish the reality of something outside the self but also the foundation for every further attempt to do so.
If this proof fails, then Descartes's hopes for human knowledge are severely curtailed, and I am stuck in
solipsism, unable to be perfectly certain of anything more than my own existence as a thinking thing.
With this reservation in mind, we'll continue through the Meditations, seeing how Descartes tried to dismantle his own reasons for doubt.
The proof of god's existence actually makes the hypothetical doubt of the First Meditation a little worse: I now know that there really is a being powerful enough to deceive me at every turn. But Descartes argued that since all perfections naturally go together, and since deception is invariably the product of imperfection, it follows that the truly omnipotent being has no reason or motive for deception. God does not deceive, and doubt of the deepest sort may be abandoned forever. (Med. IV) It follows that the simple natures and the truths of mathematics are now secure. In fact, Descartes maintained, I can now live in perfect confidence that my intellectual faculties, bestowed on me by a veracious god, are properly designed for the apprehension of truth.
But this seems to imply too much: if I have a divinely-endowed capacity for discovering the truth, then why don't I always achieve it? The problem is not that I lack knowledge of some things; that only means that I am limited. Rather, the question is why I so often make mistakes, believing what is false despite my possession of god-given mental abilities. Descartes's answer derives from an analysis of the nature of human cognition generally.
Every mental act of judgment, Descartes held, is the product of two distinct faculties: the
understanding, which merely observes or perceives, and the
will, which assents to the belief in question.
Considered separately, the understanding (although limited in scope) is adequate for human needs, since it comprehends completely everything for which it has clear and distinct ideas.
Similarly, the will as an independent faculty is perfect, since it (like the will of god) is perfectly free in every respect.
Thus, god has benevolently provided me with two faculties, neither of which is designed to produce error instead of true belief.
Yet I do make mistakes, by misusing my free will to assent on occasions for which my understanding does not have clear and distinct ideas.
For Descartes, error is virtually a moral failing, the willful exercise of my powers of believing in excess of my ability to perceive the truth.
Since the truths of reason have been restored by the demonstration of god's veracity, Descartes employed mathematical reasoning to discover the essence of bodies in the Fifth Meditation. We do not yet know whether there are any material objects, because the dream problem remains in force, but Descartes supposed that we can determine what they would be like if there were any by relying upon reason alone, since mathematics achieves certainty without supposing the reality of its objects.
According to Descartes, the essence of material substance is simply extension, the property of filling up space. (Med. V) So solid geometry, which describes the possibility of dividing an otherwise uniform space into distinct parts, is a complete guide to the essence of body. It follows that there can be in reality only one extended substance, comprising all matter in a single spatial whole. From this, Descartes concluded that individual bodies are merely modes of the one extended being, that there can be no space void of extension, and that all motion must proceed by circular vortex. Thus, again, the true nature of bodies is understood by pure thought, without any information from the senses.
By the way, this explanation of essences suggested to Descartes another proof of god's existence, a modern variation on the
Just as the essence of a triangle includes its having interior angles that add up to a straight line, Descartes argued,
so the essence of god, understood as a being in whom all perfections are united, includes necessary existence in reality.
As Descartes himself noted, this argument is no more certain than the truths of mathematics, so it also rests on the reliability of clear and distinct ideas,
secured in turn by the proofs of god's existence and veracity in the Third and Fourth Meditations.
In the Sixth Meditation, Descartes finally tried to eliminate the dream problem by proving that there is a material world and that bodies do really exist. His argument derives from the supposition that divinely-bestowed human faculties of cognition must always be regarded as adequately designed for some specific purpose. Since three of our faculties involve representation of physical things, the argument proceeds in three distinct stages. (Med. VI)
First, since the understanding conceives of extended things through its comprehension of geometrical form, it must at least be possible for things of this sort to exist. Second, since the imagination is directed exclusively toward the ideas of bodies and of the ways in which they might be purposefully altered, it is probable that there really are such things. Finally, since the faculty of sense perception is an entirely passive ability to receive ideas of physical objects produced in me by some external source outside my control, it is certain that such objects must truly exist.
The only alternative explanation for perception, Descartes noted, is that god directly puts the ideas of bodies into my mind without there acutally being anything real that corresponds to them.
(This is precisely the possibility that Malebranche would later accept as the correct account of the material world.)
But Descartes supposed that a non-deceiving god would never maliciously give me so complete a set of ideas without also causing their natural objects to exist in fact.
Hence, the bodies I perceive do really exist.
Among the physical objects I perceive are the organic bodies of animals, other human beings, and myself. So it is finally appropriate to consider human nature as a whole: how am I, considered as a thinking thing, concerned with the organism I see in the mirror? What is the true relation between the mind and the body of any human being? According to Descartes, the two are utterly distinct.
The Sixth Meditation contains two arguments in defence of Cartesian dualism: First, since the mind and the body can each be conceived clearly and distinctly apart from each other, it follows that god could cause either to exist independently of the other, and this satisfies the traditional criteria for a metaphysical real distinction. (Med. VI) Second, the essence of body as a geometrically defined region of space includes the possibility of its infinite divisibility, but the mind, despite the variety of its many faculties and operations, must be conceived as a single, unitary, indivisible being; since incompatible properties cannot inhere in any one substance, the mind and body are perfectly distinct. (Med. VI)
This radical separation of mind and body makes it difficult to account for the apparent interaction of the two in my own case. In ordinary experience, it surely seems that the volitions of my mind can cause physical movements in my body and that the physical states of my body can produce effects on my mental operations. But on Descartes's view, there can be no substantial connection between the two, nor did he believe it appropriate to think of the mind as residing in the body as a pilot resides within a ship. Although he offered several tenatative suggestions in his correspondence with Princess Elizabeth, Descartes largely left for future generations the task of developing some reasonable account of volition and sensation, either by securing the possibility of mind-body interaction or by proposing some alternative explanation of the appearances.
On the other hand, Cartesian dualism offers some clear advantages:
For one thing, it provides an easy proof of the natural immortality of the human mind or soul, which cannot be substantially affected by death, understood as an alteration of the states of the physical organism.
In addition, the distinction of mind from body establishes the absolute independence of the material realm from the spiritual, securing the freedom of scientists to rely exclusively on observation for their development of
mechanistic explanations of physical events.
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