DESCARTES, RENÉ DU PERRON (b. La Haye, Touraine, France, 31 March 1596; d. Stockholm, Sweden, 11 February 1650), natural philosophy, scientific method, mathematics, optics, mechanics, physiology.

    know the use of some machine and see some of its parts, easily infer from these the manner in which others which they have not seen are made, so, from the perceptible effects and parts of natural bodies, I have endeavoured to find out what are their imperceptible causes and parts.13

This reduction made the principles of the mechanistic model the only principles operating in nature, thus bringing the objectives of the engineer into the search for the nature of things and throwing the entire world of matter open to the same form of scientific inquiry and explanation. Research, whether into cosmology or physiology, was reduced to the discovery and elucidation of mechanisms. He could construct in distant space the imaginary world of Le monde and L'homme, and later the world of the Principles, as explicitly and unambiguously hypothetical imitations of our actual world, made in accordance with the known laws of mechanics. The heuristic power of the model was that, like any other theory advanced in anticipation of facts, its own properties suggested new questions to put to nature. The main issue in any historical judgment of Descartes here is not whether his own answers were correct but whether his questions were fruitful. In insisting that experiment and observation alone could show whether the model corresponded with actuality, he introduced further precision into his theory of demonstration.

Descartes used the word demonstrer to cover both the explanation of the observed facts by the assumed theory and the proof of the truth of the theory. When challenged with the criticism that this might make the argument circular, he replied by contrasting two kinds of hypothesis.14 In astronomy various geometrical devices, admittedly false in nature, were employed to yield true conclusions only in the sense that they “saved the appearances.” But physical theories were proposed as true. He was persuaded of the truth of the assumption that the material world consisted of particles in motion by the number of different effects he could deduce, as diverse as the operation of vision, the properties of salt, the formation of snow, the rainbow, and so on. Thus he made range of application the empirical criterion of truth. He wrote in the Discours:

If some of the matters of which I have spoken in the beginning of the Dioptrique and the Météores should, at first sight, shock people because I have called them suppositions, and do not seem to bother about their proof, let them have the patience to read them carefully right through, and I hope that they will find themselves satisfied. For it seems to me that the reasonings are so interwoven that as the later ones are demonstrated by the earlier which are their causes, these earlier ones are reciprocally demonstrated by the later which are their effects. And it must not be thought that in this I commit the fallacy which logicians call arguing in a circle, for, since experience renders the majority of these effects very certain, the causes from which I deduce them do not so much serve to prove them as to explain them; on the other hand, the causes are proved by the effects.15

The test implied precisely by the criterion of range of confirmation was the experimentum crucis. This is the most obvious feature in common between Descartes's logic of experiment and that of Francis Bacon. Descartes described its function in the Discours:

Reviewing in my mind all the objects that have ever been presented to my senses, I venture truly to say that I have not there observed anything that I could not satisfactorily explain by the principles I had discovered. But I must also confess that the power of nature is so ample and so vast and that these principles are so simple and so general, that I have observed hardly any particular effect that I could not at once recognize to be deducible from them in several different ways, and that my greatest difficulty is usually to discover in which of these ways it depends on them. In such a case, I know no other expedient than to look again for experiments [expériences] such that their result is not the same if it has to be explained in one of these ways as it would be if explained in the other.16

It was a logician rather than an experimenter who seems to have been uppermost in Descartes's application of this criterion in the same way to very general assumptions, such as the corpuscularian natural philosophy, and to questions as particular as whether the blood left the heart in systole or in diastole. Descartes argued in La description du corps humain (1648-1649) that whereas Harvey's theory that the blood was forced out of the heart by a muscular contraction might agree with the facts observed so far, “that does not exclude the possibility that all the same effects might follow from another cause, namely from the dilatation of the blood which I have described. But in order to be able to decide which of these two causes is true, we must consider other observations which cannot agree with both of them.”17 Harvey replied in his Second Disquisition to Jean Riolan.

As the great optimist of the scientific movement of the seventeenth century, Descartes habitually wrote as if he had succeeded in discovering the true principles of nature to such an extent that the whole scientific program was within sight of completion. Then, as Seth Ward neatly put it, “when the operations of nature shall be followed up to their staticall (and mechanicall) causes, the use of induction will cease, and syllogisme succeed in place of it.” But Descartes would surely have agreed with Ward's