THE EXTERNAL WORLD 67 



assuming the canons by which it has been obtained, and 

 applying them with more care and with more precision. 

 Philosophy cannot boast of having achieved such a degree 

 of certainty that it can have authority to condemn the 

 facts of experience and the laws of science. The philo- 

 sophic scrutiny, therefore, though sceptical in regard to 

 every detail, is not sceptical as regards the whole. That 

 is to say, its criticism of details will only be based upon 

 their relation to other details, not upon some external 

 criterion which can be applied to all the details equally. 

 The reason for this abstention from a universal criticism 

 is not any dogmatic confidence, but its exact opposite ; 

 it is not that common knowledge must be true, but that 

 we possess no radically different kind of knowledge 

 derived from some other source. Universal scepticism, 

 though logically irrefutable, is practically barren ; it can 

 only, therefore, give a certain flavour of hesitancy to our 

 beliefs, and cannot be used to substitute other beliefs 

 for them. 



Although data can only be criticised by other data, 

 not by an outside standard, yet we may distinguish 

 different grades of certainty in the different kinds of 

 common knowledge which we enumerated just now. 

 What does not go beyond our own personal sensible 

 acquaintance must be for us the most certain : the 

 " evidence of the senses " is proverbially the least open 

 to question. What depends on testimony, like the facts 

 of history and geography which are learnt from books, 

 has varying degrees of certainty according to the nature 

 and extent of the testimony. Doubts as to the existence 

 of Napoleon can only be maintained for a joke, whereas 

 the historicity of Agamemnon is a legitimate subject of 

 debate. In science, again, we find all grades of certainty 

 short of the highest. The law of gravitation, at least 



