In Meditation II Descartes proposes a truth that cannot be undermined by the skeptical arguments of the previous meditation. The certain truth is: "I am, I exist." Or more completely (as stated in the Discourse on Method), I think, therefore I am. Descartes recognizes that with respect to any belief that p, either he is deceived in believing p or he is not deceived. But in either case, he must exist. So the one truth that is certain is that I exist. Even if there is an all-powerful demon who controls me and implants false beliefs in my mind, he can never deceive me with respect to my own existence. My existence is a precondition for my being deceived.

But Descartes asks, "what I am?" This is crucial. For it might be possible to doubt one's own existence if one conceived of oneself in terms or under categories that could be doubted. For instance, if one thought of oneself as a human person with a particular physical structure and biological processes. One could be mistaken about all of this. The evil demon could make you believe that you are a male person with two arms and legs, etc, even though this is false. Hence, Descartes rejects the Aristotelian definition of the self as a rational animal just because it raises further questions the answers to which can be doubted. Only a conception of self as thought can circumvent being subject to doubt. Only if one conceives of oneself as thought does it follow that one's own existence is indubitable or certain, for one cannot doubt that one is a thinking thing since doubt itself presupposes thought. So the one certain truth is that I exist, I exist as a thinking thing.

Other way of seeing how it is that we can be certain of our own existence is the mind is transparent to itself in a way that the (external) world is not transparent to the mind. Detached from matters of the external world, the mind can find certainty with respect to itself. Hence, we can be certain, not just that we exist as a thinking thing, but of every truth about our own mind. More specifically, we can be certain of all our immediate states of consciousness. It seems to me that there is a car in the driveway. I feel tired. I am sad. These are all examples of truths that cannot be doubted for the same reason that one cannot doubt whether one thinks or exists as a thinking thing. Although one may be mistaken about whether there is a car in one's driveway, one can't be mistaken about it seeming to one that this is so.

Compare the two different propositions:

[A] It seems to me that there is a car in the driveway => truth about one's own mind

[B] There is a car in the driveway => truth about something external to one's own mind.

Descartes thinks that we can be certain of [A], though [B] can be doubted.

The problem generated by Meditation II is this. Even if Descartes can be certain of his own existence and all truths about his own mind, it does not follow from this that he can have certainty about the external world. But his project was precisely to provide a foundation for human knowledge, where this includes what is allegedly known in the physical and natural sciences. In short, his goal is to provide a foundation for knowledge of the external world. At this stage, though, he has provided only self-knowledge. If this is a foundation for knowledge of the external world, we are still left with the question as to how he can build knowledge of the external world on such a foundation. Meditation III attempts to address this problem and to begin to show how one can move from self-knowledge to knowledge of the external world.

Descartes' strategy is as follows:

STEP1:

Descartes first makes an observation about an important characteristic of his self-knowledge, namely that it is clear and distinct.

STEP 2:

From (A) he infers a general rule, "Everything I clearly and distinctly perceive is true." With this rule he can deduce the truth of other propositions if he clearly and distinctly perceives them.

STEP 3:

He notes a possible problem with (B), namely that there are many things which he thought were evident and clear but which turned out to be false. Moreover, on the basis of the evil demon argument the possibility of deception remains for many things that we regard as clear and distinct. So the rule or principle in (B) cannot be applied in an unproblematic way. By itself, it is insufficient to ground certainty of the external world. This will be so just if the evil demon argument remains, as long as it is possible that there is a powerful being who engages in systematic and global deception of human persons.

STEP 4:

Descartes proposes an argument that will eliminate the evil demon hypothesis. This argument is an argument for God's existence, an argument for the existence of a being who is all powerful, all-knowing, and all-good. Descartes will argue that if God exists, then the evil demon hypothesis is eliminated. If and only if this hypothesis is eliminated, we can place our trust in the veracity of clear and distinct ideas.

Descartes' basic argument for God's existence is as follows:

(P1) I have an idea of God's existence.

(P2) If I have an idea of God's existence, then God exists as the cause of my idea of God.

(C) Therefore, God exists as the cause of my idea of God.

Notice that the first premise (P1) is an truth internal to Descartes' own mind. Since Descartes is entitled only to begin with what is certain for him, he must construct his argument on the basis of truths that are internal to his own mind.

Descartes attempts to provide an argument for (P2) though. The argument is roughly as follows:

(P3) Ideas owe their existence ultimately to some reality that mirrors the idea (actually or potentially).

(P3) is supported by the following considerations. Descartes thinks that ideas must be caused by something. Moreover, ideas must be caused by something with at least as much reality as is represented by idea itself (what it is about). This is for Descartes the application of a more general principle about causality as such as applied to things in general (not simply ideas of things). With respect to any thing or substance, there must be as much reality in the cause as in the effect, and the reality of the cause must be correlated or resemble the reality of the effect. A stone, for instance, can only come into existence by something that has existed and has existed in such a way that it can produce a stone object. The cause of a stone must contain in itself stoneness (actually or potentially), otherwise it could not make a stone. In short, "you can't give what you haven't got." Descartes thinks that this principle of causality applies not merely to actual or formal existence of things (i.e., things as they really exist) but also to objective reality (i.e., things as they exist in our minds). Ideas also must be caused by something (real or another idea) that has the power to generate the idea. So in asking what can cause an idea of x, we must ask what x is. Moreover, the ultimate source of any idea must be something real, for every idea were generated from a previous idea there would be an infinite regress of ideas. But no finite mind can hold an infinite number of ideas.

(P4) No finite thing can be the cause of the idea of God.

The idea of God is the idea of an unlimited, infinite, or perfect being. Descartes argues that this idea could not have originated from ourselves. The finite cannot produce the concept of the infinite. There is more (objective) reality contained in the idea of the infinite than is contained in an idea of the finite. Human beings could only be the source of finite ideas. We could, for instance, be the source of our ideas of monkeys or stones. We could arrive at such concepts by reflecting on the concepts given in our self-knowledge. If we know ourselves as a finite substance, we can think our other substances that are finite but that differ from us in various ways. We can extrapolate these concepts as modifications of the concepts involved in our own self-knowledge. The idea of God, however, cannot be derived from ourselves, for the concept of an infinite being is not a mere modification of any concept given in our self-knowledge. For the same reason, the idea of God could not come from any finite thing, since no finite thing could contain the conceptual requirements to derive the concept of the infinite. So the concept of the infinite can only come from a reality than is itself infinite, but that would be God.

Objection. . . .

The obvious objection to Descartes' argument here is that we can derive the concept of the infinite from the concept of the finite simply by negating the latter. The infinite just means that which is not finite. But if we know ourselves as finite, surely we can modify this concept by a simply mental act of negation. We can derive the notion of the infinite from the concept of that which is not-finite, and this in turn from the concept of the finite.

Descartes' response to this is simple. He argues that we could not possess the concept of the finite unless we already possessed the idea of the infinite (at least latent in our consciousness). All recognition of limit presupposes the notion of a boundary. Anytime we recognize that we are imperfect in some way, this judgment takes place over against an assumed standard of perfection. So the recognition of the infinite is actually prior in us to the recognition of the finite. We derive the concept of finite with the help of an antecedent idea of the infinite. We do not derive the concept of the infinite from the concept of the finite.

So Descartes thinks he proves that the idea of God in us logically implies that the existence of the object of our thought. God must exist. Moreover, being all knowing, all powerful, and all good, God would not engage in systematic global deception of his creatures, nor would he allow any other creature to do this. Descartes requires deception, or permitting deception, as an indication of imperfection. Hence, God could not be involved in deceiving us, anymore than our parents would deceive us if they wished good to us.

In this way, Descartes thinks that he eliminates the evil demon hypothesis. If God exists, the evil demon hypothesis is undermined. The existence of a perfect being as the author of our existence is not compatible with our being systematically and globally deceived with respect to those things that appear to us clearly and distinctly. Hence, we can trust the veracity of our clear and distinct ideas.

First, in developing the argument for God's existence Descartes appeals to "truths" other than truths about his own mind. Although one of his premises in the basic argument is a truth about his own mind, the second one is not, nor are the other claims he makes in the subsidiary argument for (P2). At any rate, these premises are not supported by argument based exclusively on introspective beliefs. For instance, Descartes depends on a premise about cause and effect, that there is such a thing and that all ideas must have a cause, and that there must be as much formal reality in the cause as there is objective reality in the idea. These do not appear to be truths about one's own mind.

Secondly, Descartes appeals to clarity and distinctness at various points in his argument for God's existence. But presumably we are not entitled to trust our clear and distinct perceptions until such time as we have proved that God's exists. This raises a potential problem of circular reasoning. We can't trust clear and distinct ideas (beyond the cogito) until such time as we have proved God's existence. But we know that we have proved God's existence because it is clear and distinct the idea of his existence entails that he actually exists.

A. Descartes' search for certainty leads his to posit his own consciousness as the starting point for human knowledge. This initiates an important turn towards subjectivity or inwardness that characterizes much of philosophy after Descartes. There is also an interesting parallel between Descartes and Plato at this juncture. Plato emphasized the answer being within us. Similarly for Descartes, the answer to the epistemological crisis was within us, in our immediate consciousness of our own existence.

B. Although Descartes' philosophy requires a turn inwards, perspective turning in on itself, it also requires a turn outwards, a transcendent movement. For Descartes, only the existence of God can bridge the gap between knowledge of self and knowledge of the external world. Although Descartes begins with himself, he is compelled in his intellectual inquiry to move beyond himself. There is a parallel here also with Plato. In the Meno, the road of intellectual inquiry, though it involves a look inward, also must move beyond the inward. Virtue, it is concluded, is acquired by divine dispensation.

C. Descartes, unlike Plato, begins with doubt. Plato and Aristotle held the view that philosophy begins with wonder, not doubt. But doubt, in Descartes, is intimately tied to a sharp dichotomy between the inward and the outward, between a realm of epistemic safety in which knowledge is secure and a realm of epistemic doubt in which claims to knowledge are initially subject to the skeptical arguments of the Meditation I. In Descartes, this dichotomy implies a risk for the self, the risk of isolation and alienation from the external world. This risk will become an important aspect of later philosophers such as Kierkegaard, for whom the basic questions of philosophy deal with the nature of the self.