But what is ? What is ? The words and do not denote any really existing thing and therefore cannot be defined. Those words only denote a certain stage of understanding of phenomena. I do not know why a certain event occurs; I think that I cannot know it; so I do not try to know it and I talk about . I see a force producing effects beyond the scope of ordinary human agencies; I do not understand why this occurs and I talk of . To a heard of rams, that ram the herdsman dries each evening into a special enclosure to feed, and that becomes twice as fat as the others, must seem to be a genius.