"Where determinism fails, science fails." (Determinism and Physics, p.18).

In Part One, Chapter V, of his 1948 Human Knowledge: Its Scope and Limits, Russell argues for a thorough-going mechanical determinism of brain processes, but he does make a rare mention of quantum uncertainty that may be based on Arthur Stanley Eddington's ideas, which in any case was the basis for David Wiggins suggestion for an amplified quantum uncertainty.

Although quantum mechanical, it proves to be little more than the clinamen of Epicurus or the delicately balanced state of mind that James Clerk Maxwell was looking for, so that an infinitesimally small nudge of the mind could tip the material body one way or the other.

And although Russell knows the history of philosophy better than most professional philosophers, he appears blissfully unaware of the ancient and well-known criticism of chance as the direct cause of action, which eliminates moral responsibility.

From the standpoint of orthodox psychology, there are two boundaries between the mental and physical, namely, sensation and volition. "Sensation" may be defined as the first mental effect of a physical cause, "volition" as the last mental cause of a physical effect. I am not maintaining that these definitions will prove ultimately satisfactory, but only that they may be adopted as a guide in our preliminary survey. In the present chapter I shall not be concerned with either sensation or volition themselves, since they belong to psychology; I shall be concerned only with the physiological antecedents and concomitants of sensation, and with the physiological concomitants and consequents of volition. Before considering what science has to say, it will be worth while to view the matter first from a common-sense point of view.

Suppose something is said to you, and in consequence you take some action; for example, you may be a soldier obeying the word of command. Physics studies the sound waves that travel through the air until they reach the ear; physiology studies the consequent events in the ear and nerves and brain, up to the moment when you hear the sound; psychology studies the sensation of hearing and the consequent volition; physiology then resumes the study of the process, and considers the outgoing chain of events from the brain to the muscles and the bodily movement expressing the volition; from that point onward, what happens is again part of the subject matter of physics. The problem of the relation of mind and matter, which is part of the stock in trade of philosophy, comes to a head in the transition from events in the brain to the sensation, and from the volition to other events in the brain. It is thus a twofold problem: how does matter affect mind in sensation, and how does mind affect matter in volition? I do not propose to consider this problem at this stage; I mention it now only to show the relevance of certain parts of physiology to questions which philosophy must discuss.

The physiological processes which precede and accompany sensation are admirably set forth in Adrian's book The Basis of Sensation: The Action of the Sense Organs (London, 1928). As everyone knows, there are two sorts of nerve fibers, those that carry messages into the brain and those that carry messages out of it. The former alone are concerned in the physiology of sensation. Isolated nerves can be stimulated artificially by an electric current, and there is good reason to believe that the processes thus set up are essentially similar to those setup naturally in nerves that are still in place in a living body. When an isolated nerve is thus stimulated in an adequate manner, a disturbance is set up which travels along the nerve at a speed of about 220 miles an hour (100 meters a second). Each nerve consists of a bundle of nerve fibers running from the surface of the body to the brain or the spinal cord. The nerve fibers which carry messages to the brain are called "afferent"; those which carry messages from the brain are called "efferent." A nerve usually contains both afferent and efferent fibers. Broadly speaking, the afferent fibers start from sense organs and the efferent fibers end in muscles.

The response of a nerve fiber to a stimulus is of what is called the "all-or-nothing" type, like the response of a gun to pressure on the trigger. A slight pressure on the trigger produces no result, but a pressure which is sufficiently great produces a specific result which is the same however great the pressure may be (within limits). Similarly when a nerve fiber is stimulated very slightly, or for a very brief period (less than .00001 of a second), there is no result, but when the stimulus is sufficient a current travels along the nerve fiber for a very brief period (a few thousandths of a second), after which the nerve fiber is "tired" and will not transmit another current until it is rested. At first, for two or three thousandths of a second, the nerve fiber is completely refractory; then it recovers gradually. During the period of recovery a given stimulus produces a smaller response, and one which travels more slowly. Recovery is complete after about a tenth of a second. The result is that a constant stimulus does not produce a constant state of excitement in the nerve fibers, but a series of responses with quiescent periods between. The messages that reach the brain are, as Adrian puts it, like a stream of bullets from a machine gun, not like a continuous stream of water. It is supposed that in the brain, or the spinal column, there is a converse mechanism which reconverts the discrete impulses into a continuous process, but this, so far, is purely hypothetical.

Owing to the discontinuous nature of the response to a stimulus, the response will be exactly the same to a constant stimulus as to one which is intermittent with a frequency adapted to the period of recovery in the nerve. It would seem to follow that there can be no means of knowing whether the stimulus is constant or intermittent. But this is not altogether true. Suppose, for instance, that you are looking at a bright spot of light: if you could keep your eyes absolutely fixed, your sensations would be the same if the light flickered with appropriate rapidity as they would be if the light were steady. But in fact it is impossible to keep the eyes quite still, and therefore fresh, unfatigued nerves are perpetually being brought into play.

A remarkable fact, which might seem to put a limit on the informative value of sensations, is that the response of the nerve fiber is the same to any stimulus of sufficient strength and duration: there is just one message, and only one, that a given nerve fiber can transmit. But consider the analogy of a typewriter: if you press a given letter, only one result occurs, and yet the typewriter as a whole can transmit any information, however complicated. The mechanism of the efferent nerve fibers appears to be just the same as that of the afferent nerve fibers; the messages that travel from the brain to the muscles have the same jerky character as those that travel from the sense organs to the brain.

But the most interesting question remains. What goes on in the brain between the arrival of a message by the afferent nerves and the departure of a message by the efferent nerves? Suppose you read a telegram saying, "All your property has been destroyed in an earthquake," and you exclaim, "Heavens! I am ruined!" We feel, rightly or wrongly, that we know the psychological links, after a fashion, by introspection, but everybody is agreed that there must also be physiological links. The current brought into the vision center by the optic nerve must pass thence to the speech center, and then stimulate the muscles which produce your exclamation. How this happens is still obscure. But it seems clear that from a physiological point of view there is a unitary process from the physical stimulus to the muscular response. In man this process may be rendered exceedingly complex by the operation of acquired habits, especially language habits, but in some less highly organized animals the process is simpler and less difficult to study; the reason why the moth approaches the flame, for example, is fairly well understood in physiological terms.

This raises a question of great interest, namely: Is the process in the brain, which connects the arrival of the sensory stimulus with the departure of the message to the muscles, completely explicable in physical terms? Or is it necessary to bring in "mental" intermediaries, such as sensation, deliberation, and volition? Could a superhuman calculator, with sufficient knowledge of the structure of a given brain, predict the muscular response to a given stimulus by means of the laws of physics and chemistry? Or is the intervention of mind an essential link in connecting a physical antecedent (the stimulus) with a physical consequent (a bodily movement)?

Until more is known about the brain than is known at present, it will not be possible to answer this question confidently in either sense. But there are already some grounds, though not conclusive ones, for regarding what might be called the materialist answer as the more probable one. There are reflexes, where the response is automatic and not controlled by volition. From unconditioned reflexes, by the law of habit, conditioned reflexes arise, and there is every reason to regard habit as physiologically explicable. Conditioned reflexes suffice to explain a great part of human behavior; whether there is a residue that cannot be so explained must remain, for the present, an open question.

At a later stage, I shall maintain that there is no such gulf between the mental and the physical as common sense supposes. I shall also maintain that even if the physiological causal chain from sense organ to muscle can be set forth in terms which ignore the psychological occurrences in the middle of the chain, that will not prove that volitions are not "causes" in the only valid sense of the word "cause." But both of these contentions require considerable argument and elucidation. For the present, I will only add a few words from the standpoint of scientific common sense.

If — as seems likely — there is an uninterrupted chain of purely physical causation throughout the process from sense organ to muscle, it follows that human actions are determined in the degree to which physics is deterministic. Now physics is only deterministic as regards macroscopic occurrences, and even in regard to them it asserts only very high probability, not certainty. It might be that, without infringing the laws of physics, intelligence could make improbable things happen, as Maxwell's demon would have defeated the second law of thermodynamics by opening the trap door to fast-moving particles and closing it to slow-moving ones.

On these grounds it must be admitted that there is a bare possibility — no more — that although occurrences in the brain do not infringe the laws of physics, nevertheless their outcome is not what it would be if no psychological factors were involved. I say there is no more than a bare possibility for several reasons. In the first place, the hypothesis supposes only the microscopic laws preserved, not the macroscopic laws. But the evidence for the macroscopic laws is better than the evidence for the microscopic laws, and very strong grounds would be needed to justify a belief that on some occasion they had failed. In the second place, all the occurrences which illustrate the connection of mind and matter are macroscopic: a volition, for example, results in a perceptible bodily movement, not in a mere atomic change. In the third place, the study of processes in the nerves and brain, so far, has shown physical causation wherever adequate observation was possible; the region as to which there is still ignorance is one where very minute phenomena are concerned, and where observation is very difficult. There is therefore, so far, not the smallest positive reason for supposing that there is anything about physical processes in the brain that involves different macroscopic laws from those of the physics of inanimate matter.

Nevertheless, for those who are anxious to assert the power of mind over matter it is possible to find a loophole. It may be maintained that one characteristic of living matter is a condition of unstable equilibrium, and that this condition is most highly developed in the brains of human beings. A rock weighing many tons might be so delicately poised on the summit of a conical mountain that a child could, by a gentle push, send it thundering down into any of the valleys below; here a tiny difference in the initial impulse makes an enormous difference to the result.

Perhaps in the brain the unstable equilibrium is so delicate that the difference between two possible occurrences in one atom suffices to produce macroscopic differences in the movements of muscles. And since, according to quantum physics, there are no physical laws to determine which of several possible transitions a given atom will undergo, we may imagine that, in a brain, the choice between possible transitions is determined by a psychological cause called "volition." All this is possible, but no more than possible; there is not the faintest positive reason for supposing that anything of the sort actually takes place.

On the evidence as it exists the most probable hypothesis is that, in the chain of events from sense organ to muscle, everything is determined by the laws of macroscopic physics. To return to our previous illustration of the man who reads a telegram and exclaims, "I am ruined!": it seems probable that if you had a sufficiently minute knowledge of his brain structure, and if you were a sufficiently good mathematician, you could foretell that when the shapes making the message on the telegram came into his field of vision they would set up a process ending in certain movements in his mouth, to wit, those producing the sounds which we represent in writing as "I am ruined."

It is here assumed that you could make this prophecy without knowing English; it should not be necessary for you to know the meaning either of the telegram or of his exclamation. The difference between a man knowing English and a man not knowing it should, on the physiological side, consist in the presence in one case, and absence in the other, of connections between the afferent nerves when stimulated by the hearing or reading of English words and the efferent nerves producing the appropriate response. This difference we suppose visible to a hypothetical observer without his having to know the "meaning" either of the stimulus or of the response.

This hypothesis, it must be admitted, does not seem very plausible, and I am far from asserting dogmatically that it is true. The most that can legitimately be asserted, in my opinion, is that it is the right working hypothesis for a man investigating the physiological concomitants of sensation and volition. In so far as it is true, it may help him to make discoveries; if, at some point, it is false, its falsehood is most likely to be discovered by means of experiments suggested by the assumption of its truth. In so far as the hypothesis is true, physiology is a science independent of psychology; if at any point it is false, physiology ceases to be autonomous. Asa matter of practical policy, the physiologist does well to assume that his science is autonomous so long as no evidence to the contrary has been discovered.