Man comes into the world with a cry: a burst of light, a slap, initiate him into the universe of sensation. The material of science is this: our experience of the natural world, the world that is – not the ones that might be. Somehow in the mind this raw experience is ordered, and this order is the substance of science. What happens there is the application to a great diversity of the phenomena of the world of many of those elements called common sense, used every day and resting on certain suppositions we make concerning the world about us. Some of these are probably universal to man and beast. Others are more particular. We tend to accept them without special awareness, and some are so well hidden we are scarcely conscious of their existence.

The foremost supposition is the belief that the world outside ourselves, outside our own mind, exists. This belief is so primitive that it is very likely shared by all, except animals lowest on the evolutionary scale and some philosophers (whose position on the evolutionary scale we cannot guess).