INDUCTIONS IMPROPERLY SO CALLED. 333 



limit, those motions, even as now known to us, might be ex- 

 pressed with any degree of accuracy that might be required. 

 The elliptical theory, as a mere description, would have a great 

 advantage in point of simplicity, and in the consequent facility 

 of conceiving it and reasoning about it ; but it would not 

 really be more true than the other. Different descriptions, 

 therefore, may be all true : but not, surely, different explana- 

 tions. The doctrine that the heavenly bodies moved by a 

 virtue inherent 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 was 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, arid the other 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 ? Assuredly the one is true, and the other 

 absolutely false.* 



* Dr. Whewell, in his reply, contests the distinction here drawn, and main- 

 tains, that not only different 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 Discovery, p. 231) : " Un- 

 doubtedly all these explanations may be true and consistent with 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 point was 



reached, the vortex was merely a machinery, well or ill devised, for producing 



