to replace the old metaphysics with one founded on his newly formed principles, which he did inMeditationes de prima philosophia (1641; Meditations on First Philosophy, written in 1628-1629 although not published for more than ten years). This accomplished, he was free to set about his main enterprise, which was to lay bare the secret laws that govern the world of matter. In the next eight years, his progress was astonishing. However, the innumerable experiments that kept suggesting themselves required many hands. His motive in presenting the summary of his ideas in the Discourse on Method was to recruit workers.

There is evidence independent of the Discourse on Method that the first powerful conviction that lifted Descartes out of skepticism was the discovery of certain rules for the direction of the mind. In the Discourse on Method, he associates this phase of his development with the winter in Germany when he was twenty-three. It antedates his enterprise of systematic doubt and, indeed, is presupposed by it, the doubt being but the first step toward carrying through the rules.

The brilliant young mathematician was led to conclude that just as the most difficult demonstrations in geometry can be arrived at easily by a long chain of simple steps, soall things, to the knowledge of which man is competent, are mutually connected in the same way, and . . . there is nothing so far removed from us as to be beyond our reach, or so hidden that we cannot discover it, provided only we abstain from accepting the false for the true, and always preserve in our thoughts the order necessary for the deduction of one truth from another.

What Descartes had discovered, he believed, was a key that would open for humankind all the doors to knowledge. No new power of insight or of reasoning was needed—every person, essentially, has as much insight and reason as any other. The failure of previous inquirers stemmed from no natural deficiency but from clumsiness and inexperience. Chiefly, people had been seduced by the powerful claims of their senses and imagination so that the sober witness of reason had been obscured. What people needed was a set of rules that would help them to keep faithfully to truths that, once the debris had been removed, would shine upon the mind with a natural light.

Descartes was, therefore, a bold exponent of what is known as the deductive or a priori method. Still, his approach must not be confused with the Scholasticism that, in philosopher Francis Bacon’s words, “flies from the senses and particulars to the most general axioms, and from these principles, the truth of which it takes for settled and immoveable, proceeds to judgment and to the discovery of middle axioms.” The fault of the Aristotelian syllogism, Descartes said, was that, although it helps people reason persuasively about things they already think they know, it is of no help in investigating the unknown. Thus, Descartes’s method was more radically a priori than Aristotle’s, which drew its premises from induction.

Turning his back on traditional logic and taking his cue from geometry, Descartes envisaged a chain of linear inferences that would progress from an initial truth so simple and obvious as to be self-evident to a second that would be seen at once to be included in the first, and thence to a third, and so forth. In practice, the problem would always be to find the simple truth to which the chain could be anchored; afterward, all that would be necessary would be to preserve the true order. Each particular truth along the way would be entirely obvious to anyone who understood what was being affirmed—just as in arithmetic, a child who understands a sum fathoms everything that is within reach of the greatest genius who contemplates the same set of figures.

For convenience, Descartes summed up his principles in four rules:1. Never accept any idea as true that is not so clearly and distinctly true as to be beyond all possibility of being doubted. 2. Divide each complex question into simple ones. 3. Order thoughts from the simplest to the most complex. 4. Review the series of inferences to make sure there are no breaks or false links in the chain.

If these rules were rigorously followed, an obscure matter, such as the function of the lungs in the body, would be rendered perfectly intelligible. Such, at least, was the promise that inspired the youthful Descartes and launched him on his great career.

The biographical narrative and the exposition of method make up the first half of the Discourse on Method. Part 4 is an abridged version of the Meditations on First Philosophy. Descartes’s preeminently scientific interests seem somewhat incompatible with his foray into the metaphysical. However, as he explained, to have ventured at once into the difficulties of the sciences would have been contrary to right order because he regarded all such particular truths as dependent on principles borrowed from philosophy.

The investigation of the principles set forth in part 4 was “so metaphysical and uncommon” that Descartes questioned whether his readers would find it acceptable. It is, in fact, the only part of the Discourse on Method that makes serious demands upon the intelligence. Yet, because the Meditations on First Philosophy had not been published in 1637, he felt obliged to include a précis of that work. This review of the work places it in its proper perspective—as the first serious attempt to make use of the method of doubt and as the foundation for physics and physiology rather than an end in itself. Lacking this perspective, one might suppose that Descartes’s preoccupation with God and the soul stemmed from a religious interest. His Scholastic-styled proofs suggest to the modern reader that he is laboring to shore up traditional beliefs. However, when Descartes speaks of soul, he means “thinking” (and ideally “reasoning,” for all the rest of the conscious life is illusion), and when he speaks of God, he means the “ordered necessity” against which all apparent “contingency” is rendered intelligible. Therefore, far from seeking to preserve the Christian view of the world, he is substituting for it an uncompromising formulation of the presuppositions of seventeenth and eighteenth century science. Just how far his philosophical work strengthened the cause of science and speeded its development is impossible to say, but it seems likely that his metaphysics was more useful than his method.

The method required that Descartes discover a truth so simple and self-evident that it could not be further reduced and yet would verify itself. Yet every belief he considered seemed to him to be a possible error. He supposed that even his strongest convictions might have been planted in him by a malevolent and deceiving god. However, finally it struck him that even though his beliefs were mistaken and he could not go beyond the activity of doubting, the doubting itself existed; thinking existed; and if thinking, then he, as thinking substance, also existed. He expressed his conclusion in the triumphant words Cogito, ergo sum, or “I think, therefore I am.”

When he sought to discover the ground of his certainty, he found nothing that guaranteed the truth of the proposition “I think, therefore I am” other than his seeing “very clearly that in order to think it is necessary to exist.” He thus took as a general rule for further philosophical investigations the principle “that all the things which we very clearly and distinctly conceive are true.” At the same time, he admitted the difficulty of deciding which of the objects of thought were being conceived “clearly and distinctly.”

Descartes then realized that he, as one who doubted, was not perfect, and yet he had the idea of perfection and of a perfect being. He found nothing in his own experience to account for his having arrived at the idea of perfection, and he finally concluded that the idea must have been placed in him by a being possessing all the perfections he conceived, namely, by God himself. Descartes was thus led to a renewed conviction in God’s existence.

Other arguments in support of the belief in God’s existence were quickly generated. All imperfect bodies and intelligences show by their imperfection their dependency on the perfect, on God; hence, God exists. Furthermore, the idea of a perfect being entails the idea of the existence of such a being; for if a being otherwise perfect did not exist, he would lack an essential perfection. (This ontological argument, reminiscent of Saint Anselm’s more famous version, was regarded by Descartes as having the same rigorous character as a geometric proof.) Finally, Descartes decided that no belief whatsoever could be accepted as a “metaphysical certitude” except by presupposing the existence of God. Even the principle of clarity—that clear and distinct ideas are true—depended for its certainty upon God’s perfection and existence.

It has seemed to many critics of Descartes that to prove the certainty of a principle by reference to a God whose existence is proved by the use of that very same principle is certainly a circular procedure. However, Descartes probably would have argued that clear and distinct ideas are certainly true regardless of whether one knows the ground of one’s certainty. He presumed that by his arguments, he revealed the ground of all certainty: God, the perfect being, the independent support of all dependent beings and of all truths endorsed by reason.

Having reassured himself and his readers of the reliability of reason and of the method he had outlined, Descartes then proceeded to summarize the conclusions of his unpublished treatise, The World. According to his account, the treatise was a comprehensive work containing “all that . . . I thought I knew of the nature of material objects.” Beginning with light, it touched upon the sun and the stars, on the heavens, the planets, the comets, and the earth, upon terrestrial bodies, and finally, upon humans considered as organized matter. Out of caution, Descartes adopted the device of speaking about an imaginary world rather than about one created as described in the Bible. Suppose that there is matter “without form and void” in some imaginary space; if it is governed by the laws which God’s perfection has established and fixed in our minds, it must work itself out of its initial chaos and arrange itself in a manner like our heavens and our earth. Tides, air currents, mountains and seas, summer and winter, minerals and vegetables are all accounted for in terms of fixed laws of nature. One has to turn to the original work to find the laws stated. Perhaps the most significant point Descartes communicates in his summary is the important principle that the nature of anything is more easily conceived when understood in terms of its origin than when viewed simply in its completed state.

The greater part of the summary is given to human physiology. Continuing his hypothetical mode of argument, Descartes asks one to think of God as forming the body of a person exactly like one’s own but without a rational soul. This being granted, Descartes undertook to show how, according to the same laws as obtain everywhere else in nature, all the internal and external motions of the body must take place exactly as they are observed to take place, with the exception of intelligent speech and certain inventive actions. The circulation of the blood is taken as an example. Its motion through the body, as Descartes explains it, is the result of the presence in the heart of one of those fires-without-light (the kind present in processes of fermentation). The heat purifies the blood, and the resulting expansion and contraction drives it through the cavities of the heart. Hot blood warms the stomach, enabling digestion to take place. The lungs serve as condensers that cool the blood before returning it to the left ventricle. This cooling releases the animal spirits that are the “most agitated” part of the blood. These go to the brain because the arteries leading in that direction are nearly straight, and “according to the rules of Mechanics, which are the same with those of Nature . . . the weaker and less agitated parts must necessarily be driven aside by the stronger which alone in this way reach” their destination.

This example is sufficient to make clear the character of Descartes’s scientific work. However, the datedness of these theories should not obscure the original quality of Descartes’s purely mechanistic account of vital phenomena. Although he continued to hold that the mind is perfectly adjusted to the body (one’s feelings and appetites are evidence of this), he was careful to stress their complete mutual independence. As for animals other than humans, they are simply very cunningly contrived machines without any kind of mind or soul.

Descartes learned circulation of the blood not from his method but from the physician William Harvey. Like everyone else, Descartes had to combine experiences and inference from observation and experimentation in order to obtain knowledge of the existing world. However, Descartes thought that it is a mistake to begin with experiment. He believed that ordinary observation provides adequate material to arrive at the laws of nature, provided only that one reflects on it, for the germs of the most general truths exist naturally in the mind. Thus, he found no special observation necessary to account for the gross aspects of the universe—the motions of the heavenly bodies, the properties of the elements that make up the earth. However, when he came to particular problems, the situation altered. There are so many ways God might have worked that it is impossible for one to infer which is the case. In general, the laws by which a clock measures time are known; however, one must look behind the face of a particular clock to see what combinations of wheels and weights or springs the mechanic has employed. Similarly, in the case of the motion of the blood, once one has observed the arrangement of the heart with one’s eyes and felt the temperature of the blood with one’s fingers, one can understand the method that God has employed and can demonstrate by the laws of physics why the motion must take place as it does.

Experimentation became increasingly important to Descartes the further he progressed in scientific studies. However, he did not lay aside his youthful preference for demonstrative knowledge. His program for future studies, as he hinted at the close of the Discourse on Method, was to devote himself exclusively “to acquire some knowledge of Nature which shall be of such a kind as to enable us therefrom to deduce rules of Medicine of greater certainty than those at present in use.”