In the same letter Descartes described the contents of the book— and its title, which, though hardly snappy, well described what he sought to offer in it: "The Plan of a Universal Science, which is capable of raising our Nature to its Highest Degree of Perfection, together with the Optics, the Meteorology and the Geometry, in which the Author, in order to give proof of his universal Science, explains the most abstruse Topics he could choose, and does so in such a way that even persons who have never studied can understand them.2
"In this Plan [he wrote to Mersenne] I explain part of my method. I try to prove the existence of God and of the soul apart from the body, and I add many other things which I imagine will not displease the reader. In the Op tics, besides treating of refraction and the manufacture of lenses, I give detailed descriptions of the eye, of light, of vision, and of everything belonging to catoptrics and optics. In the Meteorology I dwell principally on the nature of salt, the causes of winds and thunder, the shapes of snowflakes, the colours of the rainbow—here I try also to demonstrate what is the nature of each colour—and the coronas or haloes and the mock suns or parhelia like those which appeared at Rome six or seven years ago. Finally, in the Geometry I try to give a general method of solving all the problems that have never yet been solved. All this I think will make a volume no bigger than fifty or sixty sheets. I do not want to put my name to it, as I resolved long ago; please do not say anything about it to anybody unless you judge it proper to mention it to some publishers . . ."3
The Meteorology is the most textbook-like part of the book, and was doubtless intended by Descartes to be so. The Optics is in essence a practical treatise on the manufacture of optical instruments, and Descartes went so far as to aim it at the "uneducated craftsman" on whom the production of such instruments most depended. In Le Monde Descartes' account of light had been tightly embedded in his overall physical theory, but here that theory was wholly absent.
Between them, the Meteorology and the Optics covered most of two out of the four main areas of Descartes' concerns, the other two being mathematics and metaphysics. At an early stage of planning the book Descartes was uncertain whether to include his mathematical work, and only later, when the Meteorology was already at the printers, did he decide to do so. Gaukroger plausibly conjectured that the reason for Descartes' indecision was that his active interest in mathematics had ceased some years before, and that his discoveries had been rediscovered, or anyway shared, by others since, among them Pierre de Fermat (with whom he was soon to have a bitter controversy).4
But the most important part of the book is its first part, the Discourse on Method itself. It is the first statement of Descartes' developing metaphysical ideas, and its great significance lies in its being a response to the urgent situation created by Galileo's condemnation. As Descartes recognised, the Church's censure of Galileo raised in acute form the problem of how natural science could be pursued at all, given the danger of conflict between the results of scientific investigation and the Church's teachings. He wanted to show that there was no conflict; and moreover, he wanted to prove this in the context of also showing how science could proceed without having to disguise itself as "hypotheses" about imagined worlds.
The Discourse is less fluent than the rest of the work, because it was patched together from a number of texts Descartes had written at various times over the preceding years, although he only put it into its final version after the companion essays were finished. Its most immediately striking feature is that it 'was 'written in the first person singular as if it were an essay in autobiography, all the while informally addressing the reader as "you." No doubt this form came naturally to Descartes, and he anyway had a number of explicitly autobiographical points to put in support of his "method" of enquiry; but it also served the purpose of making his reasonings easy and agreeable to follow, like listening to someone reminisce. "I have never presumed my mind to be in any way more perfect than that of the ordinary man," he rather over-modestly claimed near the beginning; "indeed I have often wished to have as quick a wit, or as sharp and distinct an imagination, or as ample or prompt a memory as some others." And yet, he continued, the method he had discovered would infallibly help all who followed it to increase their knowledge and raise it incrementally to the highest point of which their minds were capable.5
The first three sections of the Discourse are in fact purely autobiographical, describing Descartes' education and the journey of his mind to the point where he realised that, despite his excellent education at La Fleche, he needed to rethink the basis of enquiry so that he could be sure to arrive at truth. He therefore devised a set of rules to think by, enjoining the use of careful and responsible small steps always, and a set of moral principles to live by while he was seeking knowledge with the help of those rules. "In fact, I venture to say that by strictly observing the few rules I had chosen, I became very adept at unravelling . . . questions," he claimed—here he expressly meant mathematical questions, but the implication is that his method is universally efficacious.6
The fourth section anticipates, in briefer and more direct form, the famous arguments which would open the most widely studied of Descartes' philosophical works, the Meditations on First Philosophy. Two of its paragraphs encapsulate his position very succinctly and clearly. The first stated his celebrated dictum "I think therefore I am," and the supporting argument for it:
For a long time I had observed that in practical life it is sometimes necessary to act upon opinions which one knows to be quite uncertain just as if they were indubitable. But since I now wished to devote myself solely to the search for truth, I thought it necessary to do the very opposite and reject as if absolutely false everything in which I could imagine the least doubt, in order to see if I was left believing anything that was entirely indubitable. Thus, because our senses sometimes deceive us, I decided to suppose that nothing was such as they led us to imagine. And since there are men who make mistakes in reasoning, committing logical fallacies concerning the simplest questions in geometry, and because I judged that I was as prone to error as anyone else, I rejected as unsound all the arguments I had previously taken as demonstrative proofs. Lastly, considering that the very thoughts we have while awake may also occur while we sleep without any of them being at that time true, I resolved to pretend that all the things that had ever entered my mind were no more true than the illusions of my dreams. But immediately I noticed that while I was trying thus to think everything false, it was necessary that I, who was thinking this, was something. And observing that this truth / am thinking, therefore I exist was so firm and sure that all the most extravagant suppositions of the sceptics were incapable of shaking it, I decided that I could accept it without scruple as the first principle of the philosophy I was seeking.7
This deceptively simple and apparently irresistible argument has been a rich subject of debate in the philosophical tradition ever since. No one does, because no one can, doubt the truth of "I think therefore I exist"; but no one has yet given a definitive account of what kind of truth it is. If this seems surprising, note that by the terms of Descartes' own argument, one cannot treat the claim "I think therefore I exist" as—despite appearances—a simple deduction of the conclusion,"I exist," from the premise,"I think," because that would only work if there were a further premise available, a hidden major premise to the effect that "Everything that thinks, exists." But this is ruled out by the working hypothesis that all previous beliefs are false.8 Moreover, if one goes with a toothcomb over the argument, as academic philosophers do, the first problem one encounters is whether Descartes could legitimately employ the kind of sceptical doubt he used to see whether there are any indubitable beliefs left over when he had doubted everything possible.9
The sceptical considerations Descartes invoked at the outset of his quest for certainty—the fallibility of our senses, the liability to error of our reasoning powers, the hypothesis that all experience might be illusory, a mere dream—had been recognised and discus
sed by philosophers in classical antiquity from Plato onwards; but his selection and use of them in this sharply focused manner revived debate about them in a way that dominated philosophy for the next three centuries and more.
So did Descartes' statement of the point that occurred in the very next paragraph, where he committed himself to "dualism," that is, the theory that mind and matter are two distinct substances:
Next I examined attentively what I was. I saw that while I could pretend that I had no body and that there was no world and no place for me to be in, I could not for all that pretend that I did not exist. I saw on the contrary that from the mere fact that I thought of doubting the truth of other things, it followed quite evidently and certainly that I existed; whereas if I had merely ceased thinking, even if everything else I had ever imagined had been true, I should have had no reason to believe that I existed. From this I knew that I was a substance whose whole essence or nature is simply to think, and which does not require any place, or depend on any material thing, in order to exist. Accordingly this "I"—that is, the soul by which I am what I am—is entirely distinct from the body, and indeed is easier to know than the body, and would not fail to be whatever it is, even if the body did not exist.10
This famous statement of "Cartesian dualism" was the other great legacy for debate that Descartes bequeathed to philosophy. Again, the simplicity and plausibility of the argument is striking; but unlike the "I think therefore I exist" statement, which is true, the claim that minds exist independently of bodies is at least highly improbable, according to the powerful testament of our best science and our most persuasive philosophy. Whereas mental objects and events (thoughts, memories, visual experience, and the like) have no physical properties as such—for example: weight, position in space, colour, scent—all evidence suggests that they are in some sense the products of brains, and cannot exist without them.
The idea that mental life is fundamentally neurological and takes place as activity in brain structures is now the standard view. It is exceedingly well supported by all the empirical evidence, even though the problem of "qualia" (the subjective character of experience— the colours we see, the sounds we hear, the aromas we smell in the private inner world of sensation and feeling) is unsolved; for we still do not know how qualia arise in consciousness from the complex interactions of brain cells. That they do so arise is of course unquestionable; and we know a great deal about the computational activity of the brain—how it deals with information passed to it from the outside world by the senses, and how it effects its millions of finely adjusted and rapid responses to that information, most of it never apparent to our conscious minds. Optimists say that one day we will indeed understand how brains give rise to qualia in conscious experience. Pessimists think that we will never be clever enough to understand our own brains. (Happily, the pessimists have not stopped the effort to understand.)
The problem for Descartes and his immediate successors in philosophy was: how can two such utterly different things—mind and body—interact? How can a thought cause my physical arms and legs to move? How can a pin dividing my flesh cause the sensation of pain in consciousness? One can illustrate the point by analogy with a homelier question: how can one hit a baseball with a swirl of mist? Descartes at first suggested that mind and matter interacted through the pineal gland in the brain—until he realised that this was merely a way of hiding the problem inside a small inter-cranial organ.11 He ended by saying that mind-matter interaction is a mystery that only God understands.
How difficult the problem is for anyone who accepts the basic dualistic premise—that the universe contains two utterly different substances—is well illustrated by the heroic solutions proposed by two of Descartes' successors in the philosophical tradition, Malebranche and Leibniz. They both settled for the idea that mind and matter were indeed too different to interact, and therefore did not do so, but only seemed to from the viewpoint of our finite and inadequate understandings. What happens, said Malebranche, is that when God sees that something in the material realm requires a mental correlate, and vice versa, he acts to provide it; so, when your stomach is empty he causes a feeling of hunger in your mind, and a desire for a sandwich; whereupon he then causes your arms and legs to carry you to the kitchen cupboard, to remove the bread and jam, and to eat it; whereupon he next causes you to have pleasant taste sensations, and a feeling of satiety—and so on. Note that the central tenet of this view is that God is the only causal agent in the universe. Critics have observed that the drawback with this is that, even for an omnipotent and omnipresent deity, the very-large number of mind-body correlations required every moment of the average day makes for hard work; God must be an exceedingly busy being. Moreover it also requires him to be involved in the causation of a great deal of wickedness. Defenders of Malebranche replied that God was too clever to commit himself to too much hard work, and somehow arranged matters to avoid it. But as to the wickedness problem: well, here Malebranche had to attempt a delicate argument indeed, based on the idea that sin is an "intermission in activity" and therefore not the result of God's causal agency at all. This argument has not been very influential.
Leibniz's more restful (from God's point of view) solution was to say that the mental and material realms are as two clocks which have been set going in exact harmony, so that when one shows the time on its dial, the other strikes the time with its bell, making observers believe them to be connected. The drawback with this theory, however, is that it requires strict determinism in the history of the universe; everything must happen as an absolutely invariable outcome of the first setting-going by God at the beginning of time; and this means there is no free will, which in turn means that morality is an empty notion because no-one can do other than as he or she does. Furthermore, it assumes that the wickedness in the world was preordained too, and that God must be held responsible for it.
Malebranche's theory is known as "Occasionalism" because God acts to provide mental and material correlates on each occasion required. Leibniz's theory is known as "pre-established harmony" or "Parallelism," for self-explanatory reasons. The interest of these theories lies chiefly in illustrating the impasse bequeathed by Descartes to anyone who accepted his dualistic premise—which, of course, was scarcely deniable in an age where religion dominated, thus making it impossible for anyone to deny the existence of immaterial souls, a thesis which, anyway, Descartes sincerely believed.
The real solution to the mind-body mystery, however, as our best scientific investigations now tell us, is that mind and matter are not two different things at all, because there is only physical stuff in the world; which solves the interaction problem. But it does not solve the problem of consciousness.12
Descartes' Discourse and Essays, and the suppressed Le Monde which preceded it, between them have in germ all of his philosophy and science. His next two great works, published in the 1640s—the Meditations and the Principles of Philosophy (in the main, a scientific textbook)—set out the same views in more detail and with additional matter, but with no change of doctrine. So from what has thus far been said about Descartes' intellectual outlook one can see the point of his claims that "the end of all study ought to be to guide the mind to form true and sound judgements on everything that may be presented to it," and that "the sciences in their totality are but the intelligence of man; and all the details of knowledge have no value save as they strengthen the understanding."The express implication is that the purpose of philosophy—understood to mean enquiry in its most general sense—is not the mere accumulation of facts and information, not mere erudition, but the attainment of understanding, which is a thing greater than knowledge. The route to it, said Descartes, is his method.
Descartes' method was suggested to him by his early studies in geometry and arithmetic, and in particular by the fact that although he saw that these two sciences offered many truths, and gave much material for deducing further truths, they did not explain themselves sufficiently—that is, show
why their truths were true. He had in mind the fact that the discoveries of the ancient mathematicians were, by and large, peculiar to the problems they dealt with; there was no sense among the ancients of the general principles that explained any relationship between the different discoveries they made. In almost all cases, the discoveries of ancient geometry and arithmetic were triumphs of self-standing ingenuity. A way of connecting different aspects of geometry to each other, and of expressing them in a common notation which would enable discoveries to be generalised, was needed.
The means to this began to be provided at last by the mathematicians of the sixteenth century, chief among them Francois Viete, who in effect invented algebra, thus furnishing the tools for the task of generalising geometry, while at the same time savants such as Luca Pacioli, Geronimo Cardano and Niccola Tartaglia used geometrical means to solve various equations. But notation was still clumsy and unsettled, and the very names used in mathematics differed from one school or country to another. Part of Descartes' contribution was to induce some order into this confusion. He invented the now standard way of representing powers as superscript numbers (as in 102)—without a systematic way of denoting the homogeneity of successive powers the binomial theorem could not have been expressed. It is now standard to denote unknown quantities by means of lower case letters from late in the alphabet (x, y, z . . .) and known quantities from early in the alphabet (a, b, c . . .); this convention was established by Descartes.13
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