AIMS, ETC., OF SCIENTIFIC THOUGHT 97 



been considering in this particular : that it applies experience about 

 the shape of a plant which is one circumstance connected with it to 

 dealings with its medicinal properties, which are other and different 

 circumstances. Again, suppose that you had been frightened by a 

 thunder-storm on land, or your heart had failed you in a storm at sea ; 

 if any one then told you that, in consequence of this, you should al- 

 ways cultivate an unpleasant sensation in the pit of your stomach, till 

 you took delight in it that you should regulate your sane and sober 

 life by the sensations of a moment of unreasoning terror ; this advice 

 would not be an example of scientific thought. Yet it would be an 

 application of past experience to new and different circumstances. 



But you will already have observed what is the additional clause 

 that we must add to our definition in order to describe scientific 

 thought, and that only. The step between experience about animals 

 and dealings with changing humanity is the law of evolution. The 

 step from errors in the calculated places of Uranus to the existence of 

 Neptune is the law of gravitation. The step from the observed behav- 

 ior of crystals to conical refraction is made up of laws of light and 

 geometry. The step from old bridges to new ones is the laws of elas- 

 ticity and the strength of materials. 



The step, then, from past experience to new circumstances must be 

 made in accordance with an observed uniformity in the order of events. 

 This uniformity has held good in the past in certain places ; if it should 

 also hold good in the future, and in other places, then, being combined 

 with our experience of the past, it enables us to predict the future, and 

 to know what is going on elsewhere, so that we are able to regulate our 

 conduct in accordance with this knowledge. 



The aim of scientific thought, then, is to apply past experience to 

 new circumstances : the instrument is an observed uniformity in the 

 course of events. By the use of this instrument it gives us information 

 transcending our experience, it enables us to infer things that we have 

 not seen from things that we have seen ; and the evidence for the truth 

 of that information depends on our supposing that the uniformity holds 

 good beyond our experience. I now want to consider this uniformity 

 a little more closely, to show how the character of scientific thought 

 and the force of its inferences depend upon the character of the uniform- 

 ity of Nature. I cannot, of course, tell you all that is known of this 

 character without writing an encyclopaedia, but I shall confine myself 

 to two points of it, about which, it seems to me, that just now there is 

 something to be said. I want to find out what we mean when we say 

 that the uniformity of Nature is exact ; and what we mean when we 

 say that it is reasonable. 



When a student is first introduced to those sciences which have 



come under the dominion of mathematics, a new and wonderful aspect 



of Nature bursts upon his view. He has been accustomed to regard 



things as essentially more or less vague. All the facts that he has 



VOL. 11. 7 



