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astronomical system in which the earth should be 

 supposed to revolve on its own axis once in twenty- 

 four hours, but that the acceptance of this system 

 was delayed for more than a thousand years because 

 it was not compatible with the dynamical theories 

 which held the ground during that time. When a 

 more consistent dynamical syntax was put forward 

 by Galileo the objection disappeared, and the Coper- 

 nican astronomy was at once accepted by all who 

 were competent to judge of its value. It is to be 

 noted that Galileo's dynamics would have been 

 equally compatible with the Ptolemaic astronomy ; 

 the} r did not afford a proof of the Copernican theory, 

 but, by providing a dynamical syntax with which 

 this theory could be reconciled, they removed the 

 most important obstacle to its acceptance. 



Applying these illustrations to my present sub- 

 ject : I have, I hope, shown in the previous section 

 that throughout the history of modern science the 

 criterion of truth in actual use has been the one 

 which I have formulated above. I cannot offer 

 more in the way of direct proof, but perhaps 

 objections may be obviated and the proposition 

 may be made more acceptable if it can be shown 

 to be consistent with the theory of evolution that 

 such a criterion should naturally come into use 

 in the course of nervous, that is to say mental, 

 development. 



In the simplest forms of animal organism nerves 



