We can discern three distinct influences on Descartes, three conflicting world-views that fought for prominence in his day. The first was what remained of the mediaeval scholastic philosophy, largely based on Aristotelian science and Christian theology. Descartes had been taught according to this outlook during his time at Jesuit college, and this had an important influence on his work, as we shall see later. The second was the skepticism that had made a sudden impact on the intellectual world, mainly as a reaction to the scholastic outlook. This skepticism was strongly influenced by the work of the Pyrrhonians as handed down from antiquity by Sextus

Empiricus, which claimed that, \"as there is never a reason to believe \'p\' that is better than a reason not to believe \'p\', we should forget about trying to discover the nature of reality and live by appearance alone.\" This attitude was best exemplified in the work of Michel de Montaigne, who mockingly dismissed the attempts of theologians and scientists to understand the nature of God and the universe respectively. Descartes felt the force of skeptical arguments and, while not being skeptically disposed himself, came to believe that skepticism towards knowledge was the best way to discover what is certain: \"by applying skeptical doubt to all our beliefs, we can discover which of them are indubitable, and thus form an adequate foundation for knowledge.\" The third world-view resulted largely from the work of the new scientists; Galileo and Copernicus. Science had finally begun to assert itself and shake off its dated Aristotelian prejudices. Coherent theories about the world and its place in the universe were being constructed and many of those who were aware of this work became very optimistic about the influence it could have. Descartes was a \'child of the scientific revolution\', but felt that until skeptical concerns were dealt with, science would always have to contend with Montaigne and his \'cronies\', standing on the sidelines and laughing at science\'s pretenses to knowledge.

Descartes\' project, then, was to \"use the tools of the skeptic to disprove the skeptical thesis by discovering certain knowledge that could subsequently be used as the foundation of a new science, in which knowledge about the external world was as certain as knowledge about mathematics.\" It was also to \'hammer the last nail into the coffin of scholasticism\', but also, arguably, to show that God still had a vital role to play in the discovery of knowledge.

Meditation One describes Descartes\' method of doubt. By its conclusion, Descartes has seemingly subjected all of his beliefs to the strongest and most hyperbolic of doubts. He invokes the nightmarish notion of an all-powerful, demon who could be deceiving him in the realm of sensory experience, in his very understanding of matter and even in the simplest cases of mathematical or logical truths. The doubts may be obscure, but this is the strength of the method - \"the weakness of criteria for what makes a doubt reasonable means that almost anything can count as a doubt, and therefore whatever withstands doubt must be something epistemologically formidable.\"

In Meditation Two, Descartes hits upon the indubitable principle he has been seeking. He exists, at least when he thinks he exists. The cogito (Descartes\' proof of his own existence) has been the source of a great deal of discussion ever since Descartes first formulated it in the 1637 Discourse on Method, and had a great deal of misinterpretation (quite possibly as a result of Descartes\' repeated contradictions of his own position in subsequent writings). Many commentators have fallen prey to the tempting interpretation of the cogito as either syllogism or enthymeme. This view holds that Descartes asserts that he is thinking, that he believes it axiomatic that \'whatever thinks must exist\' and therefore that he logically concludes that he exists. This view, it seems to me, is wrong. In the Meditations, does Descartes write \'I am thinking, therefore I am\', nor anything directly equivalent? Rather, he says: \"Doubtless, then, that I exist, let him deceive me as he may, he can never bring it about that I am nothing, so long as I shall be conscious that I am something. So that it must, in fine, be maintained, all things being maturely and carefully considered, that this proposition I am, I exist, is necessarily true each time it is expressed by me or conceived in my mind.\" The point here is that it is impossible to doubt the truth of the proposition \'I exist\' when one utters it. It is an indubitable proposition, and one that will necessarily be \'presupposed in every attack of the skeptic\'. Descartes is not yet entitled to use syllogisms as the possibility of the \'malign demon\' is still very much alive. As an aside, Descartes himself denies that the cogito is a syllogism, although it should be mentioned that in some of the Replies to Objections he seems to assert that it is in fact a syllogism.

Finally, in the Regulae ad directionem ingenii, Descartes denies the usefulness of syllogisms as a means to knowledge. I believe that, given Descartes\' project, it is fair to grant him that the cogito deserves the status he bestows upon it. For can there be anything more certain than something that is so forceful and so powerful that every time it is presented to our mind we are forced to assent to it? What Descartes did here was to jiggle about the way philosophy normally approaches the construction of knowledge structures. By starting with self-knowledge, he elevates the subjective above the objective and forces his epistemology to rest upon the knowledge he has of his own self (and inadvertently sets the tone for the next 300 years of philosophy). This leaves him with a problem. He can know his own existence, that he is a thinking thing and the contents of his consciousness, but how can any of this ever lead to any knowledge of anything outside of himself?

The answer is that, by itself, it can\'t. Descartes, in the third Meditation, attempts to prove the existence of God, defined as a being with all perfections. This proof is to be derived from his idea of a God, defined as a being with all perfections. So far, so good - Descartes examines the contents of his consciousness and discovers within it this idea, and we can allow him this. At this point, however, he introduces a whole series of scholastic principles concerning different modes of causation and reality without proper justification: \"For, without doubt, those (ideas considered as images, as opposed to modes of consciousness) that represent substances are something more, and contain in themselves, so to speak, more objective reality, that is, participate by representation in higher degrees of being or perfection than those that represent only modes or accidents; and again the idea by which I conceive a God has certainly in it more objective reality than those ideas by which finite substances are represented.\"

\"Now it is manifest by the natural light that there must be at least as much reality in the efficient and total cause as in its effect; for whence can the effect draw its reality if not from its cause? And how could the cause communicate to it this reality unless it possessed it in itself?\" Whence do these principles draw their indubitability? Even if we grant that it is contrary to natural reason that an effect can have greater \'reality\' than its cause, that the concepts of modes and substances are coherent with Descartes\' method, let alone possess the properties that he ascribes to them, then surely we can still bring the malign demon into play? Is it not possible that this all powerful demon could bring it about that Descartes has a notion of a being with all possible perfections that he calls God? No, says Descartes, because the notion (representing something perfect) would then have more objective reality than the demon (as something evil and thus imperfect), and \'it is manifest by the natural light\' that this is not possible. But why not? Maybe the demon has just made it seem impossible, and it seems that Descartes has no answer to this.

Further problems remain. Cosmological arguments for God invoking the notion of causation have always had to contend with the problem of the cause of God. For if all events (or ideas) are caused ultimately by God, then what about God Himself? Why should He be exempt from this rule? The standard response to this is to claim that God, being omnipotent, causes Himself. One of the chief perfections that Descartes attributes to God is that of \'self-existence\', that is, that His existence depends on nothing else but itself. But if we examine this idea, it seems a little confused. If God is the efficient cause of God then we are forced to ask how something that does not yet exist can cause anything. If God is the formal cause of God, (part of the intrinsic nature of God that he exists - which seems more likely) then it seems that we have merely a reformulation of the \'ontological\' argument for God\'s existence from Meditation 5.

It seems that Descartes may have anticipated the amount of criticism that the causal proof of God would inspire, and so, after explaining how human error and a benevolent, non-deceiving God are compatible in Meditation Four, he produced in Meditation Five a version of the mediaeval ontological argument for God\'s existence. Unlike the causal argument, the ontological argument doesn\'t involve the covert import of any new principles. It simply purports to show that, from an analysis of his own idea of God, Descartes can show that He necessarily exists. The reasoning goes like this: \"I have ideas of things which have true and immutable natures. If I perceive clearly and distinctly that a property belongs to an idea\'s true and immutable nature, then it does actually belong to that nature. I perceive clearly and distinctly that God\'s true and immutable nature is that of a being with all perfections. Further, I perceive clearly and distinctly that existence is a perfection and non-existence a non- perfection. Thus existence belongs to God\'s true and immutable nature. God exists.\" One of the interesting things about this argument is that, at first sight, it does not seem to depend in any way upon anything that has been proved \'hitherto\'. It is an application of pure logic, an analysis of what we mean when we say \'God\' and a inference from that analysis. Descartes explicitly says that an idea\'s true and immutable nature does not in any way depend upon his thinking it, and thus upon his existence. Once he has perceived clearly and distinctly that an idea\'s true and immutable nature consists in such-and-such, that is the case whether or not he thinks it is, or even if he exists or not. Descartes in fact recognizes the primacy of the ontological argument: \"although all the conclusions of the preceding Meditations were false, the existence of God would pass with me for a truth at least as certain as I ever judged any truth of mathematics to be.\" If this is true, which it seems to be, then this argument is only as trustworthy as the faculties which enabled us to construct it, which are the same faculties that enable us to know mathematical truths, and so it seems worthwhile to ask how, under Descartes\' theory, we come to know mathematical truths. Descartes claims we perceive them clearly and distinctly. How do we know that what we perceive clearly and distinctly is true? Because God, being perfect, is no deceiver, and would not let it be the case that we could ever perceive something clearly and distinctly without it being the case. It seems then, that this proof of God, relying on the veracity of clear and distinct ideas, relies on the certain knowledge that a non-deceiving God exists. We have another proof of God, the causal proof as described in Meditation three. But apart from the \'patent futility\' of using one proof of \'p\' to construct another proof of \'p\', on examining the causal proof of God further, we find that it, too, relies upon a \"methodology that can only be relied upon if the divine guarantee is present, for if this guarantee is not present, then, how can we be sure that the all-powerful demon is not exercising his malignant influence?\"

This, of course, is the infamous Cartesian circle, first identified by Arnauld in the Fourth Objections and discussed ever since. Many philosophers have tried to get Descartes off the hook in various ways, some by denying that there is a circle and some by admitting the circularity but denying its significance. I will here briefly evaluate a few of their arguments.

Some commentators have taken a passage from Descartes\' reply to the Second set of Objections to indicate that Descartes is only actually interested in the \"psychological significance of fundamental truths.\" The passage is as follows:

\"If a conviction is so firm that that it is impossible for us ever to have any reason for doubting what we are convinced of, then there are no further questions for us to ask; we have everything we could reasonably want.\"

Under my interpretation, this is what it is about the cogito that makes it so important for Descartes, so we cannot have any argument with the principle expressed by him in the above passage. But can it help break the circle? When we clearly and distinctly perceive something, Descartes says, \"fairly I think, that this perception compels our assent, that we cannot but believe it. God\'s role in the system, to these commentators, is as a guarantor of our memory regarding clarity and distinctness.\" In other words, once we have proved God\'s existence, we can happily know that any memory we have of a clear and distinct idea regarding \'x\' is true (that we really did have a clear and distinct idea of \'x\'). But this does not seem satisfactory, as we still do not have a divine guarantee for the reasoning that leads us from the clear and distinct notions we originally have about God to the proof of His existence. We can give assent to the clear and distinct notions we have originally; in fact, \"we are compelled to give this assent when the notions are presented to our mind, but the logical steps we take from these ideas to the final proof is still subject to the evil demon because God is not yet proven.\" Furthermore, because these steps are needed, \"the memory of the original clear and distinct ideas are themselves subject to doubt because God is not yet proven.\" It seems that the only way either of the proofs of God could be accepted would be if we had an original clear and distinct perception of God directly presented to our mind (similar to the cogito). But this in itself would make any future proofs

redundant. Interestingly, this sounds quite similar to a divine revelation.
Harry Frankfurt, in his book \"Demons, Dreamers and Madmen\", has argued that what Descartes is actually looking for is a \"coherent, indubitable set of beliefs about the universe.\" Whether they are \'true\' or not is irrelevant. Perfect certainty is totally compatible with absolute falsity. Our certainty may not coincide precisely with \'God\'s\' truth, but should this matter?: \"Reason can give us certainty. It can serve to establish beliefs in which there is no risk of betrayal. This certainty is all we need and all we demand. Perhaps our certainties do not coincide with God\'s truth. But this divine or absolute truth, since it is outside the range of our faculties and cannot undermine our certainties, need be of no concern to us.\"

This is almost a Kantian approach to knowledge, where we as humans only concern ourselves with the phenomena of objects as they present themselves to us, not with the objects in themselves. Can we ascribe this view to Descartes? It\'s tempting, given what we have said above regarding the prime importance of indubitability, but it would seem that a God presenting ideas to us in a form which doesn\'t correspond to reality, and then giving us a strong disposition to believe that they do correspond to reality would be a deceiving God and contrary to Descartes\' notion of Him. Thus the belief set would not be coherent. Perhaps, as we do not have clear and distinct ideas of the bodies we perceive, and \"as the divine guarantee only extends as far as clear and distinct ideas\", we are being too hasty in judging that reality is how it appears to be and if we stopped to meditate further we would see that reality is actually like something else. But aside from the fact that this seems unlikely. So much for the Cartesian circle. Where does this leave the ontological argument, which we had only just begun to discuss? Aside from the methodological difficulties, there do seem to two further problems with it. The first has been noted by almost every student of Descartes over the years - that of the description of existence as a property. Put briefly, this objection states that existence is not a property like \'red\' or \'hairy\' or \'three-sided\' that can be applied to a subject, and thus it makes no sense to say that existence is part of something\'s essence. \"If we assert that x is y, we are already asserting the existence of x as soon as we mention it, prior to any application of a predicate. from the beginning. In other words, to say \'x exists\' is to utter a tautology and to say that \'x doesn\'t exist\' is to contradict oneself. So how can sentences of the form \'x doesn\'t exist\' make sense? One may well ask. It is because these sentences are shorthand for \'the idea I have of x has no corresponding reality\' and it was to solve problems like this that Bertrand Russell constructed his theory of descriptions. To add existence to an idea doesn\'t just make it an idea with a new property, it changes it from an idea into an existent entity.\"

Finally, if Descartes is right, there seems no reason why we cannot construct any other idea whose essence includes existence. For instance, if I conjure up the idea of an existent large orange building that resembles the Taj Mahal, then it is the true and immutable nature of this idea that it is a building, that this building resembles the Taj Mahal, that the building is large and purple, and that it exists. But no such building does exist, as far as I am aware, and if it did exist, its existence would not be necessary, but contingent. This in itself is enough, I think, to show that the ontological argument is false.

Once we have destroyed Descartes\' proofs of the existence of God, the edifice of knowledge necessarily comes tumbling down with them, as we find that almost everything Descartes believes in is dependent on God\'s nature as a non-deceiver:

\"I remark that the certitude of all other truths is so absolutely dependent on it, that without this knowledge it is impossible ever to know anything perfectly.\"

The only possible exceptions are those assent-compelling beliefs such as the cogito. Even these, however, are doubtful when we are not thinking about them, and the above passage does give weight to Edwin Curley\'s argument that: \"Descartes would hold that the proposition \"I exist\" is fully certain only if the rest of the argument of the Meditations goes through. We must buy all or nothing.\"
This is not the end of the story, though. As far as Descartes is concerned, by the end of Meditation Five, he has produced two powerful proofs of God, has a clear and distinct notion of his own self, has a criterion for truth, knows how to avoid error and is beginning to form ideas regarding our knowledge of corporeal bodies. And so it remains only to explain why we are fully justified in believing in corporeal bodies, and also to draw the ideas of Meditation Two regarding self-knowledge to their full conclusion.

Regarding the nature of corporeal bodies and our knowledge of them, it seems to me that, given his premises, the conclusions Descartes draws in Meditation Six are generally the correct ones. He again invokes the causal to argue that \"the ideas of bodies we have within our minds must be caused by something with at least as much formal reality as the ideas have objective reality.\" We could theoretically be producing these ideas, but Descartes dismisses this possibility for two reasons - firstly, that the idea of corporeality does not presuppose thought and secondly that our will seems to have no effect on what we perceive or don\'t perceive. (This second argument seems to me to ignore dreaming, in which what we perceive derives from us but is independent of our will). The ideas, then, could come from God, or from another being superior to us but inferior to God. But this, too, is impossible, argues Descartes, as if it were the case that \"God produces the ideas of bodies in us, then the very strong inclination we have towards believing that the idea - producing bodies resemble the ideas we have would be false and thus God would be allowing us to be deceived which is not permissible. The same would apply if any other being were producing these ideas.\"

Thus, concludes Descartes, it is most likely that our ideas of corporeal bodies are actually caused by bodies resembling those ideas. We cannot be certain, however, as we cannot claim to have clear and distinct notions of everything we perceive. We can, however, claim certainty with regard to those properties of bodies which we do know with clarity and distinction; namely, size, figure (shape), position, motion, substance, duration and number (not all of these assertions are justified). Obviously we cannot claim that we know these properties for specific bodies with clarity and distinction, for to do so would leave open the question of why it is that astronomy and the senses attribute different sizes to stars.

What Descartes means is that we can be sure that these primary qualities exist in bodies in the same way that they do in our ideas of bodies. This cannot be claimed for qualities such as heat, colour, taste and smell, of which our ideas are so confused and vague that we must always reserve judgment. I think we can grant this reasoning, with the caveat regarding dreaming that I noted above, and of course the other unproved reasonings that Descartes shows us here, such as the causal principle. Furthermore, it seems to be further proof that Descartes does believe we can get to know objects in themselves to a certain extent.

Finally, I turn to Descartes\' argument for the distinction of mind and body. Descartes believes he has shown the mind to be better known than the body in Meditation Two. In Meditation Six he goes on to claim that, as he knows his mind and knows clearly and distinctly that its essence consists purely of thought, and that bodies\' essences consist purely of extension, that he can conceive of his mind and body as existing separately. \"By the power of God, anything that can be clearly and distinctly conceived of as existing separately from something else can be created as existing separately.\" At this point, Descartes makes the apparent logical leap to claiming that the mind and body have been created separately, without justification. Most commentators agree that this is not justified, and further, that just because I can conceive of my mind existing independently of my body it does not necessarily follow that it does so. In defense of Descartes, Saul Kripke has suggested that Descartes \"may have anticipated a modern strand of modal logic that holds that if x=y, then L (x=y).\" In other words, if x is identical to y then it is necessarily identical to it. From this it follows that if it is logically possible that x and y have different properties then they are distinct. In this instance, that means that because I can clearly and distinctly conceive of my mind and body as existing separately, then they are distinct. The argument, like much modern work on identity, is too technical and involved to explore here in much depth. But it will suffice to say that we can clearly and distinctly conceive of Dr Jekyll and Mr. Hyde as being distinct and yet they are identical, necessarily so under Kripke\'s theory. It is doubtful that Kripke can come to Descartes\' aid here and Descartes needs further argument to prove that the mind and the body are distinct. And so we finish our discussion of Descartes\' attempts to extricate himself from the skeptical doubts he has set up for himself. As mentioned previously, the ultimate conclusion to draw regarding the success of the enterprise that Descartes set for himself must be that he failed. When the whole epistemological structure is so heavily dependent on one piece of knowledge - in this case the knowledge that God exists - then a denial of that knowledge destroys the whole structure. All that we can really grant Descartes - and this is certainly contentious - is that he can rightly claim that when a clear and distinct idea presents itself to his mind, he cannot but give his assent to this idea, and furthermore, that while this assent is being granted, the clear and distinct idea can be justly used as a foundation for knowledge. The most this gets us - and this is not a little - is the knowledge of our own existence each time we assert it. But Descartes\' project should not be judged by us as a failure - the fact that he addressed topics of great and lasting interest, and provided us with a method we can both understand and utilize fruitfully, speaks for itself.