INTERPRETATION 349 



reasoning that is more or less complex or prolonged. The declara- 

 tion that a conclusion is a necessary truth amounts to a claim that 

 we have reasoned correctly from premises (law or laws) that are 

 admittedly true. No matter how simple or complex the case, 

 when we declare that we have ' explained ' or ' understood ' an 

 occurrence, we mean merely that we have interpreted it in terms of 

 one or more uniformities of which we are aware. Moreover, this 

 tracing of instances and consequences to general uniformities, and 

 therefore to the qualities and relations of objects, is all that we 

 mean by 'tracing a chain of causation.' Thus, knowing the 

 common uses of pokers and that iron can be raised to a high 

 temperature by fire, we explain the hotness of a poker, we describe 

 the cause of the hotness, by saying that the poker has been in the 

 fire. On the other hand, knowing the qualities of ice, we should, 

 if we found a hot piece of it, be at a loss for an explanation. 

 Knowing that intact brains are essential to the existence of human 

 beings, we suppose we have the explanation of the death of a man 

 when we discover a bullet imbedded in his brain. We explain the 

 rise of the tide and the fall of a stone by the same law. We ex- 

 plain the rise and fall of water in a harbour by the tides of the 

 ocean, the tides by the influence of the moon, the influence 

 of the moon by the law of gravitation. We cannot as yet 

 explain the law of gravitation by reference to a wider 

 uniformity ; it is for us an ultimate law ; but between it and the 

 rise and fall of water in a harbour lies a chain of explanation, of 

 causation, and of necessary truth. When Huxley declared that, 

 " Fact I know : and law I know : but what is this Necessity, save 

 an empty shadow of my own mind's throwing ? " x he referred 

 to these ultimate uniformities which as yet we cannot explain, 

 and which perhaps we shall never be able to explain by reference 

 to more general laws. He referred to necessary truth when he 

 said " even thoughtful men usually receive with surprise the 

 suggestion, that the form of the crest of every wave that breaks, 

 wind-driven, on the sea-shore, and the direction of every particle of 

 foam that flies before the gale, are the exact effects of definite 

 causes; and, as such, must be capable of being determined, 

 deductively, from the laws of motion and the properties of air and 

 water." 2 " The very postulate of knowledge compels us to think 

 every variation and every detail, even the smallest, as so deter- 

 mined by conditions that, under the circumstances, it could not 

 possibly be other than it is. When the conditions of every detail 

 1 Essays, vol. i. p. 161. * Hume, p. 122. 



