The cause of his resigning his commission is said to have been disgust at the atrocities which he witnessed in Hungary ; but it is more likely that his object was to see the world under a different aspect, which his travelling as a private individual would enable him to do. He visited in succession Holland, France, Italy, and Switzerland, and stayed some time in Venice and Rome. It has often created surprise that while in Italy he did not visit Galileo ; and the cause which has been usually assigned was his jealousy of the fame of that father of physics—an assumption which there is reason to fear is too well founded. His repulsive conduct towards Fermat, whose overtures of an amicable correspondence he so long rejected with an appearance of disdain, seems also to intimate tho wish of Descartes to reign alone in the circle of his associates, and in the philosophic world altogether.
After completing his travels, Descartes determined to devote his attention exclusively to philosophical and mathematical inquiries; and his ambition was to renovate the whole circle of the sciences. He sold a portion of his patrimony in France, and retired to Holland, where ho Imagined he should be more free to follow his inclination without the interruptions to which his celebrity In his own country rendered him perpetually liable. His writings however involved him in much controversy, and the vivacity and dogmatism of his temper often led him to treat in a somewhat supercilious manner the greatest men amongst his contemporaries. The personal courage of Descartes was great ; and, unlike many valiant writers, he was valiant in the most trying personal dangers.
The fame of Descartes was very great, even in his lifetime; and that not only among the learned, but In the highest circles of society in every part of Europe. When therefore the church rose in arms against the heresy of his philosophy, and he was subjected to much persecution and some danger, he accepted the invitation of Christina, queen of Sweden, who offered him an asylum and complete protection from the bigoted hostility of his enemies. He was treated by the queen with the greatest distinction, and was released from the observance of any of the humiliating usages so generally exacted by sovereigns of those times from all whom they admitted into their presence. The queen however, probably from the love of differing from every one else, chose to pursue her studies with Descartes at fivo o'clock In tho morning ; and as his health was always far from robust, and now peculiarly delicate, the rigour of the climate, and the unsea sonable hour, which formed such a striking contrast with thee to which he had been many years habituated, brought ou pulmonary disease, of which he very seem expired, In the fifty-tourth year of his age. The queen wished to inter him with great huuour in Sweden ; but the Freud, ambassador interposed, and his remains were conveyed for sepulture amongst his countrymen in Paris. Thus fell one of the greatest men of his age, a victim to the absurd caprice of the royal patron under whose auspices he had taken shelter from the persecutions of the church.
Probably there is scarcely a name on record, the bearer of which has given a greater impulse to mathematical and philosophical inquiry than Descartes. As a mathematician he actually published but little, and yet in every subject which he treated ha has opened a new field of investigation. The simple application of the notation of indices to algebraical powers has totally new-modelled the whole science of algebra. The very simple conception of expressing the fundamental property of curve-lines and curve•surfaces by equations between the co-ordinates, has led to an almost total supersedence of the geometry of the ancients. The view which he proposed of the constitution of
equations is contested as to originality ; but admitting, as we do, his claims on this head to be open to dispute, the writings and discoveries of Descartes have laid the foundation for such a change in the general character of mathematical science as renders it extremely difficult for those who have not given very great attention to the older writers to follow the course of reasoning which they employed. The claims of Descartes however to the originality of his views on the composition of equations, and the relation between their roots and their coefficients, are discussed under the name of his competitor. [llaiteloTs.] His speculations in physics have often been ridiculed by subsequent writers, and there can be no doubt that they are sufficieutly absurd. Still many reasons may be urged in mitigatiou of that ridicule, and even of the more temperate censure which careful aud judicious histo rians of science have dealt out upon the intellectual character of Des cartes. It ought especially to be observed that the theories of all his predecessors were mere empirical conjectures respecting the places and paths of the celestial bodies; they constituted, so to speak, the plane astronomy of those times, in contradistinction to the physical astro nomy of ours. Those paths were not deduced as the necessary effect of sny given law of force, but as the result of some fixed and unalter able system of machinery invisible to us, and directly under either the control of original accident or the original will of God. Innumerable hypotheses of the nature of this machinery bad been framed before the time of Descartes ; and he, being dissatisfied with all others, adopted that of an ethereal fluid, which was continually revolving round a centre, like the water in a vortex. This was not so unnatural to a philosopher living before the 'Principle' made its appearance as it would be absurd in any one to contend for it now. We have indeed been too much in the habit of measuring the philosophical sanity of Descartes by the knowledge of our own times—a most unjust test to be applied to the intellectual efforts of any man by his successors. We ought rather to look to what he did accomplish under all the diffi culties of his position in respect to the then state of science, thau measure him by the efforts which were attended with no beneficial result. He was, however, the first who brought optical science under the command of mathematics, by the discovery of the law of the refraction of the ordinary ray through diaphanous bodies. He deter mined the law itself, but not as the result of any law of force. This was a later discovery : but Descartes led the way.
His iuquIries in the positive philosophy were distinguished by great acuteness and subtlety ; and though his theory has not in a dire.t form obtained many advocates in this country, it has in reality been the foundation of most of the sects which have since risen in every part of Europe. Differing as these systems do so very widely at first sight, this may be considered a paradoxical assertion. It is nevertheless the fact.
Tho works of Descartes have been collected and reprinted three times. Tho first : 1, ' Opera Omnia,' 1690-1701, 9 vols. 4to, Amst. 2, 'Opera Omnia,' 1713, also 9 vols. 4to, Amst 3, ' Opera Omuia,' 1721-26, in 13 vols. 12mo. Paris.