144 JAMES CLERK MAXWELL. [CHAP. V. 



inconsistency, it follows that the probability and credibility 

 of their testimony is increasing day by day, and the more a 

 man uses them the more he believes them. He believes 

 them. What is believing ? When the probability (there is 

 no better word found) in a man's mind of a certain proposi- 

 tion being true is greater than that of its being false, he 

 believes it with a proportion of faith corresponding to the 

 probability, and this probability may be increased or dimin- 

 ished by new facts. This is faith in general. When a man 

 thinks he has enough of evidence for some notion of his he 

 sometimes refuses to listen to any additional evidence pro or 

 con, saying, " It is a settled question, probatis prolata ; it 

 needs no evidence ; it is certain." This is knowledge as 

 distinguished from faith. He says, " I do not believe ; I 

 know." " If any man thinketh that he knoweth, he knoweth 

 yet nothing as he ought to know." This knowledge is a 

 shutting of one's ears to all arguments, and is the same as 

 " Implicit faith " in one of its meanings. " Childlike faith," 

 confounded with it, is not credulity, for children are not 

 credulous, but find out sooner than some think that many 

 men are liars. I must now to bed, so good night ; only please 

 to write when you get this, if convenient, and state the prob- 

 ability of your coming here. We perhaps will be in Edin- 

 burgh when the Wise men are there. Now you are invited 

 in a corner of a letter by 



JAMES CLERK MAXWELL. 



Glenlair, 16th September 1850. 



Professor W. Thomson has asked me 1 to make him some 

 magne-crystallic preparations which I am now busy with. 

 Now, in some of these bismuth is required, which is not to 

 be found either in Castle-Douglas or Dumfries. I have, 

 therefore, thought fit to request you, and do now request 



1 This request had probably some connection with the meeting of 

 the " wise men " in Edinburgh. Maxwell had been present, and had 

 spoken in Section A. Professor Swan remembers the surprise felt by 

 all but Forbes at seeing the beardless stripling rise to dispute some 

 point in the colour theory with Sir David Brewster. 



