REASON ENABLES US TO PREDICT. 345 



the laws and operations of nature, the more surely and 

 completely can we predict. It is by means of the great 

 laws of Kepler and Newton that men are able to foretell 

 astronomical events which will happen thousands of years 

 hence. ' It has become possible to predict, not simply that 

 under given conditions two things will always be found 

 together, but also how much of the one will be found with 

 so much of the other. It has become possible to predict, 

 not simply that this phenomenon will occur after that, but 

 the exact period of time at the end of which it will occur, or 

 the exact distance in space, or both.' l Things which, by the 

 power of inference, we know can be verified, we often do not 

 attempt to verify, and in some cases we even take every 

 means in our power to prevent their verification ; for in- 

 stance, if we know that a certain course of conduct of ours 

 is likely to produce injurious consequences, we carefully 

 avoid making the experiment. The things we know by 

 the intellect are more certain than those we know by the 

 senses, however distinct and powerful the sensory impres- 

 sion may be; because intellectual ideas are the impres- 

 sions of the senses, corrected by comparison, judgment, 

 and inference. Eeason is therefore the basis of wisdom, the 

 source of safety in probability, and the very guide of life. 



Sound scientific inference consists, in all cases, in pass- 

 ing from one proposition to another, which is either equal 

 to the whole or a part of the former, but does not exceed 

 it. Whatever is affirmed or denied of an entire class or 

 thing may, of course, be affirmed or denied of any portion 

 of that class or thing, because the whole includes the por- 

 tion. In the process of inference we also apply the prin- 

 ciple of substitution of like for like ; thus we say similar 

 causes have similar effects. 



The scientific truths upon which we reason may possess 



1 Spencer, Principles of Psychology, p. 434. 



