MEASUREMENTS 211 



is a matter of mathematical demonstration, and our mental 

 attitude is knowledge. The same may be said of Galileo s 

 conviction of the truth of the Copernican theory. All 

 knowledge is, indeed, based on beliefs, and on beliefs which 

 vary in strength as they are brought nearer to knowledge by 

 the continued application of the method of agreement 

 only. The belief in the efficacy of a drug is greatly strength 

 ened by repeated experience of its good effects, but it 

 remains an empirical belief until converted into knowledge 

 by scientific demonstration. 



When we turn our attention to subjective beliefs, we find 

 neither the same progress in certainty nor the same cul 

 mination. Whether experience has any effect in strength 

 ening them is a question which we need not consider, but 

 it is plain that they can never become knowledge, for the 

 reason that scientific demonstration, which is the condition 

 of knowledge, is unattainable. This, however, does not 

 prevent our holding them with the same degree of certainty 

 as if they were scientifically proved. Moreover, all know 

 ledge is ultimately based on belief, that is to say, on the 

 evidence of our senses. Seeing is believing : we believe in 

 the existence of a tree, but we have no knowledge of it. 

 Again, by far the greater part of our knowledge is dependent 

 on belief in another way. Most of us, if we know that the 

 earth goes round the sun, know it only because we believe 

 that the authority from which we learn that it has been 

 scientifically demonstrated is at the same time truthful 

 and competent. As Hobbes says, He that takes up con 

 clusions on the trust of authors, and doth not fetch them 

 from the first items in every reckoning, does not know any 

 thing, but only believeth. 1 The relative degrees of cer 

 tainty with which knowledge or belief appeal to us is a point 

 which is determined partly by the original constitution of 

 the individual mind, and still more, perhaps, by its training. 

 Women are more strongly impressed by beliefs than men, 

 and men with a highly developed moral or religious sense 

 than others who are not similarly endowed ; but there are 

 1 Leviathan. Of Man, III. 35 (Molesworth s edition). 

 O 2 



