1. THE TWO SETS OF MIND-BODY PROBLEMS
THERE are two mind-body problems; or, if it be
preferred, two distinct sets of such problems. The
one set is concerned with the action of body on mind and
the other with the action of mind on body. Both are
equally important; both have attracted men's occasional
attention at least since the days of Aristotle; both have
become somewhat prickly subjects, liable to arouse
violent emotions among the adherents of the many
"isms" that isolate philosophers from each other; both
have been discussed far too much in the spirit of the debating room and far too little in the spirit of scientific research.
Philosophers, it must be remembered, seek to perfect
the instruments of knowledge rather than to find knowledge itself. This is why philosophical method, fruitful
perhaps in the fields of ethics and logic, has led to nothing
but rival theories about the nature of reality. Everything
that is known about the world in which we live has been
discovered by the application of scientific method. It is
high time that mind-body problems were also subjected
to scientific method. But at present, as I have pointed out
in the Preface, scientists become a little impatient when
their attention is drawn to any of these problems; they
tend to shy away from them.
The set of problems that is concerned with the action
of body on mind is associated with that part of the central
nervous system that physiologists have called the afferent
system. This connects the organs of sense perception with
the brain. Happenings that stimulate the eyes, the ears,
the fingertips, in fact all happenings that constitute sense
data, cause messages to pass through the afferent nervous
system to the brain. These messages are appreciated by
the mind as perceptions. By the time they have become
conscious, physical data have been translated into mental
ones, waves of a suitable frequency into the sensation of
light, vibrations in the air into the sensation of sound,
molecular movements into the sensation of heat, a
molecular bombardment of the finger-tips into the
sensation of touch, chemical structure into the sensation
The translation is thorough; the distinction between
the reality discovered in physics and the reality revealed
to our senses is radical. There is but one world and yet
it can be described in two languages that seem to have
no relation to each other. Hence the first set of mind-body problems. How is the translation from the language of
physics to the language of sense data performed? Why do
we not appreciate the world around us as physicists know
it, light as waves in space, sound as vibrations of the air,
the flavour of sugar as a configuration of atoms in a
chemical structure? Why this strange difference between
the subjective world of percepts and the objective world
that is described in physics? Why indeed is there such a
thing as a subjective world? What meaning can we give
to the concept subjectivity? We know from experience
that there is such a thing as subjectivity, but we have
not yet successfully fitted it into the jigsaw puzzle
In the process of translation from objective to subjective
reality the body does something to the mind. What is it?
Could an answer to this question be found a step would
be taken towards a solution of the first of the two sets of
mind-body problems, the set that might conveniently be
called the Problem of Perception.
This problem is important and belongs rightly to the
domain of science, unwilling though scientists have been
to admit it to their domain. But it is not the one that I
propose to discuss here. I have to limit my field of enquiry
and have decided, on this occasion, to limit it to the
other set of problems, to the set that is concerned with the
action of mind on body. This set of problems is associated
in physiology, not with the afferent, but with the efferent
The efferent nervous system is connected between the
brain and the muscles. It is concerned, not with perception, but with activity. Voluntary contraction of a muscle
occurs only when a stimulus passes from the brain
through efferent nerves to the muscle. And as the stimulus
that is applied to the efferent nerves occurs as the result of
mental activity, as the result of a thought, a wish, we
observe in the stimulation of efferent nerves the action
of mind on body. The many questions that arise from
this observation make up together what can conveniently
be called the Problem of Control. The Problem of
Perception and the Problem of Control jointly constitute
the larger Problem of Interaction.
There are plenty of theories in which it is claimed that
this problem has been disposed of. Some are idealistic,
some materialistic, some agnostic. In some it is declared
that the problem has been solved, in some that there is
no problem to solve, in some that this is not a proper
problem for scientists to tackle. Like all the philosophical
"isms" that flourish side by side, these theories only
remain plausible so long as sundry true and relevant
facts are ignored; facts of physiology, such as the way
nerves and muscles work: facts of physics, such as the
principle of conservation of energy: facts of common
experience, such as that people do think, that things are
sometimes done with a purpose, that specifications are
sometimes followed. The idealists make out an excellent
and convincing case for their theories by ignoring the
physiological and physical facts. And the materialists
are able to appear very scientific to those who do not
notice how lightly they dismiss all the facts of subjective
(and not a few of objective) significance. A question that
introduces any of the ignored facts is all that is needed
to show that those are equally naive who claim to have
solved the problem and who claim that there is no problem
to solve. This is why most of those who discuss the problem
do not like questions; they prefer answers.
Nevertheless, be it popular or not, questions must be
asked with the most uncompromising clarity. I will not
apologise for saying so again and again. Nothing makes
for looser thinking than answers to questions that have
not been formulated but are only implied. This is why
I have set myself the task of putting into words all the
difficulties that are inherent in the Problem of Control,
of drawing attention to the disturbing facts that make an
easy solution impossible, of asking all the awkward
questions that I can think of. I am well aware that in the
no-man's-land where the present subject has hitherto
been discussed it is considered reprehensible to formulate
a problem without offering the balm of a theory. But
yet I have no choice but to embark on this disappointing
course. The most I shall be able to do will be to suggest
which field might be usefully explored by those who would
seek a solution. But whether the solution will be found
in that field, or in another, or whether it will never be
found, I cannot say.
One result of the prevalent disinclination to ask questions
is that there is not even a suitable language in which the
questions can be clearly formulated. So I have either to
use cumbersome, vague and ambiguous terms or coin a
few new ones with a precise meaning. I prefer the latter
course and the most important of the words that I shall
use as new technical terms have been already defined
in Chapter IV. But definitions are not very informative
and illustrations are. So I shall introduce the new terms
with the help of an example. (When I say an example
I mean an example. I do not mean an analogy. It is
rather important that this be realised. For in philosophy
analogies may serve a useful purpose. But they can only
mislead in scientific argument.)
I shall choose as my example an occasion when a mind
is doing something quite ordinary, when no creative act
is being performed, no particular skill is manifest, no high
degree of intelligence displayed. It is all too common
to illustrate the action of mind on matter by reference to
the plays of Shakespeare or the discoveries of Newton.
When this is done the impression is created that the
subject can only be properly discussed on a sublime level,
that the Problem of Control arises only when the controlled implement is a pen and does not arise when the
controlled implement is a shovel. And such an impression
is very misleading. The problem arises from facts of
2. AN ILLUSTRATION OF DIATHESIS
Let, therefore, my illustration be drawn from a place
where rough work is being done. I shall choose a foundry.
This is a large rectangular building. At the end of it a
heavy casting is being picked up by a travelling crane and
transported to the other end where it is to be deposited
on to a waiting railway truck. The crane girder spans the
width of the foundry. It moves from end to end of the
building on rails, which are set on stanchions ranged
along each wall. Other rails at right angles to these are
carried by the crane girder. A wheeled truck, called a
crab, travels on them. While the whole girder can travel
from end to end of the foundry the crab can travel along
the girder from side to side. It can, by a combination of
the two movements, be brought above any part of the
floor. The crab carries a drum on which steel wire hoisting
cables are wound. From these hangs the great crane hook.
The casting is held by a sling of hemp rope suspended
from the hook.
There are separate electric motors for movement of
the girder, movement of the crab and rotation of the
drum. Adjacent to each motor are devices known as
contactors, by means of which the current to the motor
can be switched on or off. These devices are operated by
powerful electro-magnets, the current for which is
provided by a battery of electric accumulators. The
switches for energising the electro-magnets are in a control
cabin, so placed that it commands a good view of all that
goes on in the foundry. Rheostats for control of the speed
of each motor are also placed in the control cabin.
Now let us ask the foreman what causes the casting to
move from its initial position at one end of the foundry
to its final position on the waiting railway truck at the
Surely, it may be said, the answer is very simple. But
is it? The answer is easy certainly, but not quite so simple.
For there are two distinct answers and neither is correct
unless it be completed by the other. The foreman is well
able to give both. But he may give one only.
He may say that the cause is a supply of electrical
energy from the local power station. This is true, but I
doubt if he will say it. I think he will say that the cause
is a craneman, whom we can meet and see at work if we
care to climb up into the control cabin. The power
station provides the energy, the craneman the control.
And both are needed to ensure that the casting shall reach
its specified position.
In this instance the function of each is easy to state.
The function of the energy is to ensure that the casting
shall move at all, the function of the control to ensure that
it shall move on to the waiting railway truck as called for
in the written instructions that have been sent to the fore-
man. Because of these instructions philosophers would say
that the movement of the casting was a teleological
process. They would call the energy the vis a tergo and
they would refer to the instructions, in -not the happiest
of phrases, as the final cause.
In our foundry the crane cabin is connected to the
crane motors by sundry wires and electrical devices. The
purpose of these wires and devices is well known. It is to
enable the craneman to control the movement of the
casting; the control, one can say without doing violence
to the use of language, passes through the wires and
devices to the crane motors. And control is but a particular
example of the more comprehensive concept to which I
have given the name diathesis. I propose to show that
this is something much more important than is often
realised. So long as one concentrates one's attention
exclusively on the energy required for a process one
may readily convince oneself that there is no such thing
as a mind-body problem. But when one has become aware
of the contribution made to a process by diathesis one
can no longer succeed in ignoring it.
So let us seek to understand the nature of diathesis, to
explore the paths along which it can be traced, to know
by what to detect its presence, to discover the laws to
which it conforms, if possible to make quantitative state-
ments about it. Let us do with it what one ought to do
with every proven reality and admit it into the domain of
science; let us give a name to the study of diathesis. I
A good way of revealing the reality and significance of
diathesis is to speak of it as of a commodity, just as one
speaks of energy. I am well aware that there are logical
objections to speaking thus both about energy and about
diathesis; but it would be bad exposition to hold up the
train of thought that brings this useful concept while a
discussion is going on about the way it ought to be talked
of. So I shall have to delay a justification for my manner
of speaking about diathesis until suitable occasions arise.
This question of method will be discussed more fully in
Chapter XVI, by which time its nature will have become
more apparent than it can be at this moment.
The first thing to be done is to distinguish clearly
between diathesis and energy. Let us commence our
study, therefore, by comparing the paths along which,
respectively, the energy and the diathesis reach the casting
in our foundry. These paths are shown diagrammatically
in Fig. 1. They are admittedly very much oversimplified in
the figure and only a few of the stages are labelled
through which each commodity passes. In fact each path
receives sundry tributaries not shown on the diagram.
These are important and will be discussed later. It can
be done more profitably after the distinction between the
two main paths has been firmly grasped.
3. THE PATH FOR THE ENERGY
There is so much misunderstanding of the nature of
energy among non-physicists that I propose to discuss
in some detail the path for the energy before I say anything
more about diathesis.
On its way to the foundry the energy has followed a long
and winding course. It reached its destination finally
through electric cables and its immediate source is the
electric power station. But it did not originate there.
Where and when it did originate no one can say. The
earliest knowledge we have of it is that, millions of years
ago, it was in the sun. There it existed as the potential
energy of protons and electrons. As a result of a complex
process with many intermediate stages these protons and
electrons came into combination to form helium nuclei.
In the process some of their potential energy was converted
Undergoing on its way many vicissitudes, many
conversions and reconversions from one form of energy to
another, this radiation escaped from the sun's deep
interior to the surface. Thereafter a minute fraction
arrived, after an eight minutes' journey, on our earth.
A minute fraction, again, of this radiation fell on the
green leaves of those giant forest trees that grew in bygone
geological times. Some of the radiant energy was absorbed
by the leaves in a process known as photo-synthesis. In
this process carbon atoms, contained in molecules of
carbon dioxide in the air, were separated from the oxygen
atoms and stacked in the tissues of the plant. The energy
required to disrupt the carbon dioxide molecules was
obtained from the energy in the radiation that fell on the
leaves. This energy reappeared as potential energy in the
carbon compounds into which the carbon entered after
it had been secured in the tissues of the plant.
These compounds reached the trunk and branches of
the trees and the energy was stored there. When the
trees had died, fallen, and become covered by layer after
layer of soil, their substance gradually changed to coal.
But the energy did not change. It remained in its chemical
form as the energy stored in carbon, about 11,000 British
Thermal Units to every pound. Long, long afterwards
men dug some of the coal out of the ground and brought
it to the power station where it was burnt in the furnaces.
In the power station it has undergone numerous changes
in quick succession. Its first conversion was from chemical
energy in the coal to radiant energy in the hot flue gases.
As water circulated in the boiler tubes the water became
hot and turned to steam. The radiant energy passed into
this and appeared as heat and pressure.
Passing through the turbines the steam lost both some
of its high temperature and some of its pressure. It left
the turbines at a temperature of about 1000F. and a
pressure of about one thirtieth of atmospheric pressure.
In this condition it contained rather less energy than when
it was in the boilers. The difference appeared first in
mechanical form as torque in the turbine shaft and then
as the electrical energy supplied from the station.
4- THE PATH FOR THE DIATHESIS
Now for the path for the diathesis. The source of the
diathesis is as unknown as that of the energy. It is what
we are hoping to discover. The first stage along its path
that can at present be defined with any confidence is in
the craneman's brain. As every physiologist knows, this
is in control of his nerves; his nerves are in control of the
muscles of his hands; and these are in control of the small
switches and rheostats in the crane cabin. From there
the diathesis is carried by light electrical currents to the
electro-magnets, which are, in turn, in control of the contactors. The contactors are, lastly in control of the
The above is a very rapid and superficial survey of the
path for the diathesis. It will have to be surveyed very
much more thoroughly in due course. Suffice it to note
for the moment that this path is distinct from the path for
the energy, that it is a true path, and that it permits the
passage of something as real as energy. The organisation
in the foundry depends on this path as much as on the
one that brings its supply of energy. A broken wire
between the crane cabin and the contactors or a severed
nerve in the craneman's arm would interrupt the flow
of diathesis. And after the interruption the organisation
in the foundry would be impaired. What is commonly
called order depends on an adequate supply of diathesis.
It is strange how little this is appreciated in the no-man's-land where men write for or against belief in
interaction. The arm-chair philosophers who indulge
their thoughts in that place never seem able to distinguish
between energy and what they call order. They speak
always as though order and organisation depended on a
supply of energy only. Some of them go so far as to say
that organisation is energy. In the no-man's-land where
men like to indulge in theory spinning one word is as
good as another. It is too much bother to pause and
consider whether there is any difference between energy
This confusion is the seed of many of the weeds that
grow so profusely in the no-man's-land. That it should
occur to laymen, who do not know what, in science,
energy is, might be expected. But that scientists should be
equally confused, as many of them are, is surprising indeed.
The reason for the confusion is, no doubt, that those
who write on philosophical aspects of science rarely think
about foundries and 'similar places where the distinction
between the path for the energy and the path for the
diathesis is quite obvious. They dwell for choice on more
abstract examples in which the distinction can more
easily be overlooked.
5. DIATHESIS IN HUMAN AFFAIRS
Our foundry is, of course, neither a recondite nor a
rare example of a place where energy and diathesis both
contribute to a process. Both have contributed before
in many of the places that lie along the path for the energy.
I have already mentioned that Fig. 1 is oversimplified.
Tributaries of diathesis ought to be shown there meeting
the path for the energy in sundry places. They ought to
be shown in all places where men have done something
to control the direction taken by the energy.
Such tributaries occur in the coal mine, where the coal,
with its store of energy, is hewn, loaded on trucks, brought
to the bottom of the shaft, and then raised to the surface.
They occur in the railway goods yard, where the coal
laden wagons are marshalled; in the boiler, turbine, and
switch houses of the power station. From the moment
when the coal was disturbed in its resting place deep
under the earth the path for the energy has been doubly
determinate. Its course has depended partly on diathesis
supplied to it by men and women. Were it not for their
contribution the energy would not have reached the
foundry. See Fig. 2.
In human affairs diathesis is indeed an important
commodity. Without it civilised man could not survive.
And neither could uncivilised man. Without diathesis
there would be no clothes, no houses, no books, no cooked
meals. For all these things can only come about as the
result of a controlled operation. They cannot result from
a supply of energy without a supply of diathesis. Yes, and
without diathesis there would be no machines. Strange,
therefore, that a philosopher should ever quote machines
as proof that there is no such thing as diathesis, as in
effect the philosophers of the mechanist school do.
Our factories, our shipyards, our offices, our council
chambers depend on a copious and continuous supply
of diathesis. It reaches its objectives by paths that are
sometimes quite short and sometimes much longer than
the distance from the crane cabin to the crane motors in
a foundry. Some of these paths span the oceans, so that
a person in London may control a process in New York.
Thus does the human race weave incessantly over the
earth a close meshed network consisting of paths for
diathesis. And were one to follow any path back towards
its source one would always reach a place similar to the
one reached in our foundry, namely a human brain.
Sometimes it would be a powerful brain, sometimes a
very ordinary one. It matters not. If the brain is doing
no more than control a simple and insignificant movement
that brain is passing diathesis through the efferent nervous
Even the foundry accounts provide evidence of this.
The cost of the casting when it leaves the foundry is made
up partly of the cost of the energy and partly of the cost
of the diathesis expended on it. The energy is used to
melt the metal and move the materials. It is, in this
example, the smaller part of the total cost. The greater
part is the cost of the diathesis expended on the casting
by managers, office staff and workers. A measure of
diathesis, though not a very scientific one, is man-hours.
And these are in no sense a measure of energy.
Moreover, what appears in the foundry accountant's
books as the cost of electrical energy appears mostly in
the books of the electricity supply undertaking as the cost
of diathesis. For only a small part of this cost is represented
by the cost of the coal in which the energy reached the
power station. Even when the energy supplied to an
electric power station is obtainable for nothing, as in a
hydro-electric station, the electricity is little, if anything,
cheaper than when produced in a thermal station. For
most of the cost is in sundry wages bills. So, in turn, is
the cost of the coal delivered from the mine the cost of
the diathesis expended in raising it out of the ground. The
commodity that men buy and sell in this world is always
diathesis. Energy is not bought or sold. It comes to us,
literally, as a free gift from the sky. For its source is the
light and heat that the sun, without making any charge
for it, sheds generously on the earth.
The energy can be measured in kilowatt hours, or foot
pounds, or British Thermal Units, or any other units.
The effort made by managers, office staff, bus conductors,
dentists, cooks, singers, cannot be measured in any such
units. No scientific units for the measurement of diathesis
have ever been set up. Perhaps it will be done some day.
But one is handicapped in discussing diathesis quantitatively, or in any way at all, so long as the belief remains
prevalent that diathesis is a form of energy.
So let me explain, for the benefit of those likely to be
misled by the amateur philosophers, that Newton's brain
contained less energy than a medium sized lump of coal.
And moreover, its expenditure of energy was at much the
same rate soon after his death as it had been before. But
its expenditure of diathesis occurred at a prodigious rate
while he was alive and ceased completely with his death.
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