12 LECTURE I. 



In the application of induction the greatest caution and circumspection 

 are necessary ; for it is obvious that, before we can infer with certainty 

 the complete similarity of two events, we must be perfectly well assured 

 that we are acquainted with every circumstance which can have any rela- 

 tion to their causes. The error of some of the ancient schools consisted 

 principally in the want of sufficient precaution in this respect ; for although 

 Bacon is, with great justice, considered as the author of the most correct 

 method of induction, yet, according to his own statement, it was chiefly the 

 guarded and gradual application of the mode of argument that he laboured 

 to introduce. He remarks that the Aristotelians, from a hasty observa- 

 tion of a few concurring facts, proceeded immediately to deduce universal 

 principles of science and fundamental laws of nature, and then derived 

 from these, by their syllogisms, all the particular cases which ought to 

 have been made intermediate steps in the inquiry. Of such an error we 

 may easily find a familiar instance. We observe that, in general, heavy 

 bodies fall to the ground unless they are supported ; it was therefore con- 

 cluded that all heavy bodies tend downwards ; and since flame was most 

 frequently seen to rise upwards, it was inferred that flame was naturally 

 and absolutely light. Had sufficient precaution been employed in observ- 

 ing the effects of fluids on falling and on floating bodies, in examining the 

 relations of flame to the circumambient atmosphere, and in ascertaining 

 the specific gravity of the air at different temperatures, it would readily 

 have been discovered that the greater weight of the colder air was the 

 cause of the ascent of the flame, flame being less heavy than air, but yet 

 having no positive tendency to ascend. And, accordingly, the Epicureans, 

 whose arguments, as far as they related to matter and motion, were often 

 more accurate than those of their cotemporaries, had corrected this error ; 

 for we find in the second book of Lucretius a very just explanation of the 

 phenomenon. 



" See with what force yon river's crystal stream 

 Resists the weight of many a massy beam. 

 To sink the wood the more we vainly toil, 

 The higher it rebounds, with swift recoil. 

 Yet that the beam would of itself ascend 

 No man will rashly venture to contend. 

 Thus too the flame has weight, though highly rare, 

 Nor mounts but when compelled by heavier air." 



It may be proper to notice here those axioms which are denominated by 

 Newton * rules of philosophizing ; although it must be confessed that they 

 render us very little immediate assistance in our investigations. The 

 first is, that " no more causes are to be admitted as existing in nature than 

 are true and sufficient for explaining the phenomena to be considered :" 

 the second, " therefore effects of the same kind are to be attributed, as far 

 as is possible, to the same causes :" thirdly, " those qualities of bodies which 

 cannot be increased nor diminished, and which are found in all bodies 

 within the reach of our experiments, are to be considered as general 



* PHncipia ; Introduction to Book III. 



