238 THE PRINCIPLES OF SCIENCE. [chap. 



that, although he had never made trial of the stones 

 which compose the church of Magdeburg, yet he had 

 not the least doubt that all of them were heavy, and 

 would fall if unsupported. But he adds, that it would 

 be extremely difficult to give any satisfactory explanation 

 of this confident belief^ The fact is, that the belief ought 

 not to amount to certainty until the experiment has been 

 tried, and in the meantime a slight amount of uncer- 

 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;^ 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 

 ever will 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 know- 

 ledge as to put us beyond the reach of mistake.^ 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 extensive qualification, 

 our predictions will always be subject to the chance of 

 falsification by some unexpected event, such as the division 

 of Biela's comet or the interference of an unknown gravi- 

 tating body. 



' Euler's Letters to a German Princess, translated by Hunter. 

 2nd od., vol. ii. pp. 17, 18. 



'^ Lavoisier's Chemistry, translated by Kerr. 3rd ed., pp. 114, 

 121, 123. 



* Euler's Letters, vol. ii. p. 21. 



