To realize this no very recondite instrument of research is required. We need nothing more elaborate than the method by which we are hourly in the habit of estimating each other's thoughts, and of gauging one another's motives. When I see a man laugh I assume that he is pleased; when he frowns I assume that he is angry. There is here only an application of the generally accepted maxim that when we see identical results we are warranted in assuming identical causes. In this way we can either argue from causes to effects or from effects to causes. A further statement of the same principle is that when we are dealing with biological facts we may assume that identical structures imply identical functions. The structure of a dead animal will tell us what its functions were when living as certainly as though we had the living animal in front of us. We may relate function to structure or structure to function. And in this we are using nothing more uncommon than the accepted principle of universal causation. Now, in all thinking there are two factors. There is the animal or human brain, the organ of thought, and there is the material for thought as represented by the existing knowledge of the world. If we had an exact knowledge of the kind of brain that functioned, and the exact quantity and quality of the knowledge existing, the question as to the ideas which would result would be little more than a problem in mathematics. We could make the calculation with the same assurance that an astronomer can estimate the position of a planet a century hence. In the case of primitive mankind we do not possess anything like the exact knowledge one would wish, but we do know enough to say in rather more than a general way the kind of thinking of which our earliest ancestors were capable, and what were its products. We can get at the machinery of the primitive brain, and can estimate its actions, and that without going further than we do when we assume that primitive man was hungry and thirsty, was pleased and angry, loved and feared. And, indeed, it was because he experienced fear and pleasure and love and hate that the gods came into existence.