Perhaps the scope and purpose of this and the foregoing chapters may be made clearer by showing their bearing upon certain popular beliefs which may seem self-evident but are really in my opinion either false or likely to lead to falsehood. I shall confine myself to objections which have actually been made to me when trying to explain the philosophical outcome of modern physics.[8]
[8] These objections are quoted (with kind permission) from a letter written to me by a well-known engineer, Mr. Percy Griffith, who is also a writer on philosophical subjects.
“We cannot conceive of movement apart from some thing as moving.” This is, in a sense, a truism; but in the sense in which it is usually meant, it is a falsehood. We speak of the “movement” of a drama or piece of music, although we do not conceive either as a “thing” which exists complete at every moment of the performance. This is the sort of picture we must have in our minds when we try to conceive the physical world. We must think of a string of events, connected together by certain causal connections, and having enough unity to deserve a single name. We then begin to imagine that the single name denotes a single “thing”, and if the events concerned are not all in the same place, we say the “thing” has “moved.” But this is only a convenient shorthand. In the cinema, we seem to see a man falling off a skyscraper, catching hold of the telegraph wires, and reaching the ground none the worse. We know that, in fact, there are a number of different photographs, and the appearance of a single “thing” moving is deceptive. In this respect, the real world resembles the cinema.
In connection with motion one needs to emphasise the very difficult distinction between experience and prejudice. Experience, roughly, is what you see, and prejudice is what you only think you see. Prejudice tells you that you see the same table on two different occasions; you think that experience tells you this. If it really were experience, you could not be mistaken; yet a similar table may be substituted without altering the experience. If you look at a table on two different occasions, you have very similar sensations, and memory tells you that they are similar; but there is nothing to show that one identical entity causes the two sensations. If the table is in a cinema, you know that there is not such an entity, even though you can watch it changing with apparent continuity. The experience is just like that with a “real” table; so in the case of a “real” table also, there is nothing in the actual experience to show whether there is a persistent entity or not. I say, therefore: I do not know whether there is a persistent entity, but I do know that my experiences can be explained without assuming that there is. Therefore it can be no part of legitimate science to assert or deny the persistent entity; if it does either, it goes beyond the warrant of experience.
The following is a verbally cited passage in the letter referred to objecting to what was said above about “force”:
“The concept of Force is not of physical but of psychological origin. Rightly or wrongly it arises in the most impersonal contemplation of the Stellar Universe, where we observe an infinite number of spherical bodies revolving on their own axes and gyrating in orbits round each other. Rightly or wrongly, we naturally conceive of these as having been so constituted and so maintained by some Force or Forces.”
We do not, in fact, “observe” what it is here said that we observe; all this is inferred. What we observe, in astronomy, is a two-dimensional pattern of points of light, with a few bright surfaces of measurable size when seen through the telescope (the planets), and of course the larger bright surfaces that we call the sun and moon. Most of this pattern (the fixed stars) rotates round the earth once in every twenty-three hours and fifty-six minutes. The sun rotates in varying periods, which average twenty-four hours and never departs very far from the average. The moon and planets have apparent motions which are more irregular. These are the observed facts. There is no logical impossibility about the formulæ doctrine of spheres rotating round the earth, one for each planet and one for the stars. The modern doctrines are simpler, but not one whit more in accordance with observed facts; it is our passion for simple laws that has made us adopt them.
The last sentence of the above quotation raises some further points of interest. “Rightly or wrongly”, the writer says, “we naturally conceive of these as having been so constituted and so maintained by some Force or Forces.” I do not deny this. It is “natural”, and it is “right or wrong”—more specifically, it is wrong. “Force” is part of our love of explanations. Everyone knows about the Hindu who thought that the world does not fall because it is supported by an elephant, and the elephant does not fall because it is supported by a tortoise. When his European interlocutor said “But how about the tortoise?” he replied that he was tired of metaphysics and wanted to change the subject. “Force”, as an explanation, is no better than the elephant and the tortoise. It is an attempt to get at the “why” of natural processes, not only at the “how”. What we observe, to a limited extent, is what happens, and we can arrive at laws according to which observable things happen, but we cannot arrive at a reason for the laws. If we invent a reason, it needs a reason in its turn, and so on. “Force” is a rationalising of natural processes, but a fruitless one since “force” would have to be rationalised also.
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.