So far I have been trying to exhibit some of the features which are apt to characterize litigations between non-rival theories or lines of thought by examining some rather special and localized issues. You will have felt, I expect and hope, that the fatalist dilemma, Zeno's dilemma, and my puzzles about pleasure are all, though in different ways, somewhat peripheral or marginal tangles—tangles whose unravelling does not promise by itself to lead to the unravelling of the tangles that really matter, save in so far as it may be instructive by example. Henceforward I shall be discussing a spider's-web of logical troubles which is not away in a corner of the room, but out in the middle of the room. This is the notorious trouble about the relations between the World of Science and the Everyday World.

We often worry ourselves about the relations between what we call ‘the world of science’ and ‘the world of real life’ or ‘the world of common sense’. Sometimes we are even encouraged to worry about the relations between ‘the desk of physics’ and the desk on which we write.

When we are in a certain intellectual mood, we seem to find clashes between the things that scientists tell us about our furniture, clothes and limbs and the things that we tell about them. We are apt to express these felt rivalries by saying that the world whose parts and members are described by scientists is different from the world whose parts and members we describe ourselves, and yet, since there can be only one world, one of these seeming worlds must be a dummy-world. Moreover, as no one nowadays is hardy enough to say ‘Bo’ to science, it must be the world that we ourselves describe which is the dummy-world. Before directly confronting this issue, let me remind you of a partly parallel issue which, though it exercised our great-grandfathers and grandfathers, does not any longer seriously exercise us.

When Economics was entering its adolescence as a science, thinking people were apt to feel themselves torn between two rival accounts of Man. According to the new, tough-minded account presented by the economists, Man was a creature actuated only by considerations of gain and loss—or at least he was this in so far as he was enlightened.