274 THE PRINCIPLES OF SCIENCE. 



tainty enters, because we cannot be sure that the stones of 

 the Magdeburg church resemble other stones in all their 

 properties. 



In like manner, not one of the inductive truths which 

 men have established, or think they have established, is 

 really safe from exception or reversal. Lavoisier, when 

 laying the foundations of chemistry, met with so many 

 instances tending to show the existence of oxygen in 

 all acids, that he adopted a general conclusion to that 

 effect, and devised the name oxygen accordingly. He 

 entertained no appreciable doubt that the acid existing 

 in sea salt also contained oxygen a ; yet subsequent ex- 

 perience falsified his expectations. 



This instance refers to a science in its infancy, speaking 

 relatively to the possible achievements of men. But all 

 sciences are and wiU ever remain in their infancy, relatively 

 to the extent and complexity of the universe which they 

 undertake to investigate. Euler expresses no more than 

 the truth when he says that it would be impossible to fix 

 on any one thing really existing, of which we could have 

 so perfect a knowledge as to put us beyond the reach of 

 mistake k. 



Like remarks may be made concerning all other in- 

 ductive inferences. We may be quite certain that a comet 

 will go on moving in a similar path if all circumstances 

 remain the same as before ; but if we leave out this exten- 

 sive qualification, our predictions will always be subject 

 to the chance of falsification by some wholly unexpected 

 event, such as the division of Biela's comet, or the un- 

 foreseen interference of some planetary or other gravitating 

 body. 



Inductive inference might attain to certainty if our 



a Lavoisier's 'Chemistry/ translated by Kerr. 3rd edit. pp. 114, 121, 

 123. b Euler's 'Letters,' vol. ii. p. 21. 



