334 INDUCTION. 



In every way, therefore, it is evident that to explain in 

 duction as the colligation of facts by means of appropriate 

 conceptions, that is, conceptions which will really express 



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 doctrine, 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 reference 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 indi 

 cates, 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 dis 

 tinction which Mr. Mill attempts to draw between descriptions like Kepler s 

 law of elliptical orbits, and other examples of induction.&quot; 



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

 that the planets moved in the same 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 Descrip 

 tion ; it would, no doubt, have been reconcileable with the Newtonian theory. 

 The vortices, however, were not a mere aid to conceiving the motions of the 

 planets, 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. Ac 

 cording to Descartes 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 New 

 tonian theory does not absolutely contradict any hypothesis which may be framed 

 respecting the mode of their production. The Newtonian theory, regarded as a 

 mere description of the planetary motions, does not ; but the Newtonian 

 theory as an explanation of them does. For in what does the explanation con 

 sist ? 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 towards 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 impossible that both 

 opinions can be true. As well might it be said that there is no contradiction 



