INDUCTIONS IMPROPERLY SO CALLED. 219 



in their celestial nature; the doctrine that they were moved by impact 

 (which led to the hypothesis of vortices as the only impelling force capable 

 of whirling bodies in circles), and the Newtonian doctrine, that they are 

 moved by the composition of a centripetal with an original projectile 

 force ; all these are explanations, collected by real induction from supposed 

 parallel cases ; and they were all successively received by philosophers, as 

 scientific truths on the subject of the heavenly bodies. Can it be said of 

 these, as wis said of the different descriptions, that they are all true as far 

 as they go? Is it not clear that only one can be true in any degree, and 

 the otlier two must be altogether false? So much for explanations: let us 

 now compare different predictions : the first, that eclipses will occur when 

 one planet or satellite is so situated as to cast its shadow upon another; 

 the second, that they will occur when some great calamity is impending 

 over mankind. Do these two doctrines only differ in the degree of their 

 truth, as expressing real facts with unequal degrees of accuracy? Assur- 

 edly the one is true, and the other absolutely false.* 



* Dr. Whewell, in his reply, contests tlie distinction here drawn, and maintains, that not 

 only diHerent descriptions, but different explanations of a phenomenon, may all be true. Of 

 the three theories respecting the motions of the heavenly bodies, he says (Philosophy of Dis- 

 covery, p. 231): " Undoubtedly all these explanations may be true and consistent w^th each 

 other, and would be so if each had been followed out so as to show in what manner it could 

 be made consistent with the facts. And this was, in reality, in a great measure done. The 

 doctrine that the heavenly bodies were moved by vortices was successfully modified, so that 

 it came to coincide in its results with the doctrine of an inverse-quadratic centripetal force. 



When this jjoiiit was reached, the vortex was merely a machinery, well or ill devised, 



for producing such a centripetal force, and therefore did not contradict the doctrine of a cen- 

 tripetal force. Newton himself does not appear to have been averse to explaining gravity by 

 impulse. So little is it true that if one theory be true the other must be false. The attempt 

 to explain gravity by the impulse of streams of particles flowing through the universe in all 

 directions, which I have mentioned in the Philosophy, is so far from being inconsistent with 

 the Newtonian theory, that it is founded entirely upon it. And even with regard to the doc- 

 trine, that the heavenly bodies move by an inherent virtue ; if this doctrine had been main- 

 tained in any such way that it was brought to agree with the facts, the inherent virtue must 

 have had its laws determined ; and then it would have been found that the virtue had a refer- 

 ence to the central body ; and so, the 'inherent virtue' must have coincided in its effect with 

 the Newtonian force ; and then, the two explanations would agree, except so far as the word 

 'inherent' was concerned. And if such a part of an earlier theory as this word inherent in- 

 dicates, is found to be untenable, it is of course rejected in the transition to later and more 

 exact theories, in Inductions of this kind, as well as in what Mr. Mill calls Descriptions. 

 There is, therefore, still no validity discoverable in the distinction which Mr. Mill attempts to 

 draw between descriptions like Kepler's law of elhptical orbits, and other examples of induc- 

 tion." 



If the doctrine of vortices had meant, not that vortices existed, but only that the planets 

 moved in the saine manner as if they had been whirled by vortices ; if the hypothesis had been 

 merely a mode of representing the facts, not an attempt to account for them ; if, in short, it 

 had been only a Description ; it would, no doubt, have been reconcilable with the Newtonian 

 theory. The vortices, however, were not a mere aid to conceiving the motions of the plan- 

 ets, but a supposed physical agent, actively impelling them ; a material fact, which might be 

 true or not true, but could not be both true and not true. According to Descartes's theory it 

 was true, according to Newton's it was not true. Dr. Whewell probably means that since the 

 phrases, centripetal and projectile force, do not declare the nature but only the direction of 

 the forces, the Newtonian theory does not absolutely contradict any hypothesis which may be 

 framed respecting the mode of their production. The Newtonian theory, regarded as a raei-e 

 description of the planetary motions, does not ; but the Newtonian theory as an explanation 

 of them does. For in what does the explanation consist? In ascribing those motions to a 

 general law which obtains between all particles of matter, and in identifying this with the law 

 by which bodies fall to the ground. If the planets are kept in their orbits by a force which 

 draws the particles composing them toward every other particle of matter in the solar system, 

 they are not kept in those orbits by the impulsive force of certain streams of matter which 

 whirl them round. The one explanation absolutely excludes the other. Either the planets 

 are not moved by vortices, or they do not move by a law common to all matter. It is im- 



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