When it is said, as it often is, that “force” belongs to the world of experience, we must be careful to understand what can be meant. In the first place, it may be meant that calculations which employ the notion of force work out right in practice. This, broadly speaking, is admitted: no one would suggest that the engineer should alter his methods, or should give up working out stresses and strains. But that does not prove that there are stresses and strains. A British medical man renders his accounts in guineas, although they have long since ceased to exist except as a name; he obtains a real payment, though he employs a fictitious coin. Similarly, the engineer is concerned with the question whether his bridge will stand: the fact of experience is that it stands (or does not stand), and the stresses and strains are only a way of explaining what sort of bridge will stand. They are as useful as guineas, but equally imaginary. But when it is said that force is a fact of experience, there is something quite different that may be meant. It may be meant that we experience force when we experience such things as pressure or muscular exertion. We cannot discuss this contention adequately without going into the relation of physics to psychology, which is a topic we shall consider at length at a later stage. But we may say this much: if you press your finger-tip upon a hard object, you have an experience which you attribute to your finger-tip, but there is a long chain of intermediate causes in nerves and brain. If your finger were amputated you could still have the same experience by a suitable operation on the nerves that formerly connected the finger with the brain, so that the force between the finger-tip and the hard object, as a fact of experience, may exist when there is no finger-tip. This shows that force, in this sense, cannot be what concerns physics. As the above example illustrates, we do not, in fact, experience many things that we think we experience. This makes it necessary to ask, without too much assurance, in what sense physics can be based upon experience, and what must be the nature of its entities and its inferences if it is to make good its claim to be empirically grounded. We shall begin this inquiry in the next chapter. CHAPTER XII PHYSICS AND PERCEPTION