38§ HISTORY OF PHYSICAL ASTRONOMY. 



were patiently laboring to bring the mechanical problem of the uni- 

 verse into its most distinct form, in order that it might be solved at 

 last and forever. 



I do not mean to assert that Descartes borrowed his doctrines from 

 Kepler, or from any of his predecessors, for the theory was sufficientlv 

 obvious ; and especially if we suppose the inventor to seek his sugges- 

 tions rather in the casual examples offered to the sense than in the 

 exact laws of motion. Nor would it be reasonable to rob this philos- 

 opher of that credit, of the plausible deduction of a vast system from 

 apparently simple principles, which, at the time, was so much admired ; 

 and' which undoubtedly was the great cause of the many converts to 

 his views. At the same time we may venture to say that a system of 

 doctrine thus deduced from assumed principles by a long chain of 

 reasoning, and not verified and confirmed at every step by detailed 

 and exact facts, has hardly a chance of containing any truth. Des- 

 cartes said that he should think it little to show how the world is con- 

 structed, if he could not also show that it must of necessity have been 

 so constructed. The more modest philosophy which has survived the 

 boastino-s of his school is content to receive all its knowledge of facts 

 from experience, and never dreams of interposing its peremptory must 

 be when nature is ready to tell us what is. The a priori philosopher has, 

 however, always a strong feeling in his favor among men. The deduc- 

 tive form of his speculations gives them something of the charm and 

 the apparent certainty of pure mathematics ; and while he avoids that 

 laborious recurrence to experiments, and measures, and multiplied ob- 

 servations, which is irksome and distasteful to those who are impatient 

 to grow wise at once, every fact of which the theory appears to give 

 an explanation, seems to be an unasked and almost an infallible wit- 

 ness in its favor. 



My business with Descartes here is only with his physical Theory 

 of Vortices ; which, great as was its glory at one time, is now utterly 

 extinguished. It was propounded in his Principia Philosophic?, in 

 1644. In order to arrive at this theory, he begins, as might be ex- 

 pected of him, from reasonings sufficiently general. He lays it down 

 as a maxim, in the first sentence of his book, that a person who seeks 

 for truth must, once in his life, doubt of all that he most believes. Con- 

 ceiving himself thus to have stripped himself of all his belief on all 

 subjects, in order to resume that part of it which merits to be retained, 

 he begins with his celebrated assertion, " I think, therefore I am ;" 

 which appears to him a certain and immovable principle, by means of 



