THE GROWTH OF TRUTH 7 



Secondly, all scientific truth is conditioned by the 

 state of knowledge at the time of its announcement. 

 Thus, at the beginning of the seventeenth century, the 

 science of optics and mechanical appliances had not 

 made possible (so far as the human mind was con- 

 cerned) the existence of blood capillaries and blood 

 corpuscles. Jenner could not have added to his Inquiry 

 a discourse on immunity ; Sir William Perkin and the 

 chemists made Koch possible ; Pasteur gave the condi- 

 tions that produced Lister ; Davy and others furnished 

 the preliminaries necessary for anaesthesia. Every- 

 where we find this invariable filiation, one event follow- 

 ing the other in orderly sequence ' Mind begets mind,' 

 as Harvey says; 'opinion is the source of opinion. 

 Democritus with his atoms, and Eudoxus with his chief 

 good, which he placed in pleasure, impregnated Epi- 

 curus; the four elements of Empedocles, Aristotle; 

 the doctrine of the ancient Thebans, Pythagoras and 

 Plato; geometry, Euclid' (De Generatione}. 



And, thirdly, to scientific truth alone may the homo 

 mensura principle be applied, since of all mental treasures 

 of the race it alone compels general acquiescence. That 

 this general acquiescence, this aspect of certainty, is 

 not reached per saltum, but is of slow, often of difficult, 

 growth marked by failures and frailties, but crowned 

 at last with an acceptance accorded to no other product 

 of mental activity is illustrated by every important 

 discovery from Copernicus to Darwin. 



The growth of Truth corresponds to the states of 

 knowledge described by Plato in the Theaetetus acquisi- 

 tion, latent possession, conscious possession. Scarcely 

 a discovery can be named which does not present these 

 phases in its evolution. Take, for example, one of the 

 most recent : Long years of labour gave us a full know- 



