evidence that causal laws have held hitherto, at least in the observed
portions of the past? This question must not be confused with the
further question: Does this evidence warrant us in assuming the truth of
causal laws in the future and in unobserved portions of the past? For
the present, I am only asking what are the grounds which lead to a
belief in causal laws, not whether these grounds are adequate to support
the belief in universal causation.
The first step is the discovery of approximate unanalysed uniformities
of sequence or coexistence. After lightning comes thunder, after a blow
received comes pain, after approaching a fire comes warmth; again, there
are uniformities of coexistence, for example between touch and sight,
between certain sensations in the throat and the sound of one's own
voice, and so on. Every such uniformity of sequence or coexistence,
after it has been experienced a certain number of times, is followed by
an expectation that it will be repeated on future occasions, _i.e._ that
where one of the correlated events is found, the other will be found
also. The connection of experienced past uniformity with expectation as
to the future is just one of those uniformities of sequence which we
have observed to be true hitherto. This affords a psychological account
of what may be called the animal belief in causation, because it is
something which can be observed in horses and dogs, and is rather a
habit of acting than a real belief. So far, we have merely repeated
Hume, who carried the discussion of cause up to this point, but did not,
apparently, perceive how much remained to be said.
Is there, in fact, any characteristic, such as might be called causality
or uniformity, which is found to hold throughout the observed past? And
if so, how is it to be stated?
The particular uniformities which we mentioned before, such as lightning
being followed by thunder, are not found to be free from exceptions. We
sometimes see lightning without hearing thunder; and although, in such a
case, we suppose that thunder might have been heard if we had been
nearer to the lightning, that is a supposition based on theory, and
therefore incapable of being invoked to support the theory. What does
seem, however, to be shown by scientific experience is this: that where
an observed uniformity fails, some wider uniformity can be found,
embracing more circumstances, and subsuming both the successes and the
failures of the previous uniformity. Unsupported bodies in air fall,
unless they are balloons or aeroplanes; but the principles of mechanics
give uniformities which apply to balloons and aeroplanes just as
accurately as to bodies that fall. There is much that is hypothetical
and more or less artificial in the uniformities affirmed by mechanics,
because, when they cannot otherwise be made applicable, unobserved
bodies are inferred in order to account for observed peculiarities.
Still, it is an empirical fact that it is possible to preserve the laws
by assuming such bodies, and that they never have to be assumed in
circumstances in which they ought to be observable. Thus the empirical
verification of mechanical laws may be admitted, although we must also
admit that it is less complete and triumphant than is sometimes
supposed.
Assuming now, what must be admitted to be doubtful, that the whole of
the past has proceeded according to invariable laws, what can we say as
to the nature of these laws? They will not be of the simple type which
asserts that the same cause always produces the same effect. We may take
the law of gravitation as a sample of the kind of law that appears to be
verified without exception. In order to state this law in a form which
observation can confirm, we will confine it to the solar system. It then
states that the motions of planets and their satellites have at every
instant an acceleration compounded of accelerations towards all the
other bodies in the solar system, proportional to the masses of those
bodies and inversely proportional to the squares of their distances. In
virtue of this law, given the state of the solar system throughout any
finite time, however short, its state at all earlier and later times is
determinate except in so far as other forces than gravitation or other
bodies than those in the solar system have to be taken into
consideration. But other forces, so far as science can discover, appear
to be equally regular, and equally capable of being summed up in single
causal laws. If the mechanical account of matter were complete, the
whole physical history of the universe, past and future, could be
inferred from a sufficient number of data concerning an assigned finite
time, however short.
In the mental world, the evidence for the universality of causal laws is
less complete than in the physical world. Psychology cannot boast of any
triumph comparable to gravitational astronomy. Nevertheless, the
evidence is not very greatly less than in the physical world. The crude
and approximate causal laws from which science starts are just as easy
to discover in the mental sphere as in the physical. In the world of
sense, there are to begin with the correlations of sight and touch and
so on, and the facts which lead us to connect various kinds of
sensations with eyes, ears, nose, tongue, etc. Then there are such facts
as that our body moves in answer to our volitions. Exceptions exist, but
are capable of being explained as easily as the exceptions to the rule
that unsupported bodies in air fall. There is, in fact, just such a
degree of evidence for causal laws in psychology as will warrant the
psychologist in assuming them as a matter of course, though not such a
degree as will suffice to remove all doubt from the mind of a sceptical
inquirer. It should be observed that causal laws in which the given term
is mental and the inferred term physical, or _vice versa_, are at least
as easy to discover as causal laws in which both terms are mental.
It will be noticed that, although we have spoken of causal laws, we have
not hitherto introduced the word "cause." At this stage, it will be well
to say a few words on legitimate and illegitimate uses of this word. The
word "cause," in the scientific account of the world, belongs only to
the early stages, in which small preliminary, approximate
generalisations are being ascertained with a view to subsequent larger
and more invariable laws. We may say, "Arsenic causes death," so long as
we are ignorant of the precise process by which the result is brought
about. But in a sufficiently advanced science, the word "cause" will not
occur in any statement of invariable laws. There is, however, a somewhat
rough and loose use of the word "cause" which may be preserved. The
approximate uniformities which lead to its pre-scientific employment may
turn out to be true in all but very rare and exceptional circumstances,
perhaps in all circumstances that actually occur. In such cases, it is
convenient to be able to speak of the antecedent event as the "cause"
and the subsequent event as the "effect." In this sense, provided it is
realised that the sequence is not necessary and may have exceptions, it
is still possible to employ the words "cause" and "effect." It is in
this sense, and in this sense only, that we shall intend the words when
we speak of one particular event "causing" another particular event, as
we must sometimes do if we are to avoid intolerable circumlocution.
III. We come now to our third question, namely: What reason can be given
for believing that causal laws will hold in future, or that they have
held in unobserved portions of the past?
What we have said so far is that there have been hitherto certain
observed causal laws, and that all the empirical evidence we possess is
compatible with the view that everything, both mental and physical, so
far as our observation has extended, has happened in accordance with
causal laws. The law of universal causation, suggested by these facts,
may be enunciated as follows:
"There are such invariable relations between different events at the
same or different times that, given the state of the whole universe
throughout any finite time, however short, every previous and subsequent
event can theoretically be determined as a function of the given events
during that time."
Have we any reason to believe this universal law? Or, to ask a more
modest question, have we any reason to believe that a particular causal
law, such as the law of gravitation, will continue to hold in the
future?
Among observed causal laws is this, that observation of uniformities is
followed by expectation of their recurrence. A horse who has been driven
always along a certain road expects to be driven along that road again;
a dog who is always fed at a certain hour expects food at that hour and
not at any other. Such expectations, as Hume pointed out, explain only
too well the common-sense belief in uniformities of sequence, but they
afford absolutely no logical ground for beliefs as to the future, not
even for the belief that we shall continue to expect the continuation of
experienced uniformities, for that is precisely one of those causal laws
for which a ground has to be sought. If Hume's account of causation is
the last word, we have not only no reason to suppose that the sun will
rise to-morrow, but no reason to suppose that five minutes hence we
shall still expect it to rise to-morrow.
It may, of course, be said that all inferences as to the future are in
fact invalid, and I do not see how such a view could be disproved. But,
while admitting the legitimacy of such a view, we may nevertheless
inquire: If inferences as to the future _are_ valid, what principle must
be involved in making them?
The principle involved is the principle of induction, which, if it is
true, must be an _a priori_ logical law, not capable of being proved or
disproved by experience. It is a difficult question how this principle
ought to be formulated; but if it is to warrant the inferences which we
wish to make by its means, it must lead to the following proposition:
"If, in a great number of instances, a thing of a certain kind is
associated in a certain way with a thing of a certain other kind, it is
probable that a thing of the one kind is always similarly associated
with a thing of the other kind; and as the number of instances
increases, the probability approaches indefinitely near to certainty."
It may well be questioned whether this proposition is true; but if we
admit it, we can infer that any characteristic of the whole of the
observed past is likely to apply to the future and to the unobserved
past. This proposition, therefore, if it is true, will warrant the
inference that causal laws probably hold at all times, future as well as
past; but without this principle, the observed cases of the truth of
causal laws afford no presumption as to the unobserved cases, and
therefore the existence of a thing not directly observed can never be
validly inferred.
It is thus the principle of induction, rather than the law of causality,
which is at the bottom of all inferences as to the existence of things
not immediately given. With the principle of induction, all that is
wanted for such inferences can be proved; without it, all such
inferences are invalid. This principle has not received the attention
which its great importance deserves. Those who were interested in
deductive logic naturally enough ignored it, while those who emphasised
the scope of induction wished to maintain that all logic is empirical,
and therefore could not be expected to realise that induction itself,
their own darling, required a logical principle which obviously could
not be proved inductively, and must therefore be _a priori_ if it could
be known at all.
The view that the law of causality itself is _a priori_ cannot, I think,
be maintained by anyone who realises what a complicated principle it is.
In the form which states that "every event has a cause" it looks simple;
but on examination, "cause" is merged in "causal law," and the
definition of a "causal law" is found to be far from simple. There must
necessarily be _some a priori_ principle involved in inference from
the existence of one thing to that of another, if such inference is ever
valid; but it would appear from the above analysis that the principle in
question is induction, not causality. Whether inferences from past to
future are valid depends wholly, if our discussion has been sound, upon
the inductive principle: if it is true, such inferences are valid, and
if it is false, they are invalid.
IV. I come now to the question how the conception of causal laws which
we have arrived at is related to the traditional conception of cause as
it occurs in philosophy and common sense.
Historically, the notion of cause has been bound up with that of human
volition. The typical cause would be the fiat of a king. The cause is
supposed to be "active," the effect "passive." From this it is easy to
pass on to the suggestion that a "true" cause must contain some
prevision of the effect; hence the effect becomes the "end" at which the
cause aims, and teleology replaces causation in the explanation of
nature. But all such ideas, as applied to physics, are mere
anthropomorphic superstitions. It is as a reaction against these errors
that Mach and others have urged a purely "descriptive" view of physics:
physics, they say, does not aim at telling us "why" things happen, but
only "how" they happen. And if the question "why?" means anything more
than the search for a general law according to which a phenomenon
occurs, then it is certainly the case that this question cannot be
answered in physics and ought not to be asked. In this sense, the
descriptive view is indubitably in the right. But in using causal laws
to support inferences from the observed to the unobserved, physics
ceases to be _purely_ descriptive, and it is these laws which give the
scientifically useful part of the traditional notion of "cause." There
is therefore _something_ to preserve in this notion, though it is a very
tiny part of what is commonly assumed in orthodox metaphysics.
In order to understand the difference between the kind of cause which
science uses and the kind which we naturally imagine, it is necessary to
shut out, by an effort, everything that differentiates between past and
future. This is an extraordinarily difficult thing to do, because our
mental life is so intimately bound up with difference. Not only do
memory and hope make a difference in our feelings as regards past and
future, but almost our whole vocabulary is filled with the idea of
activity, of things done now for the sake of their future effects. All
transitive verbs involve the notion of cause as activity, and would have
to be replaced by some cumbrous periphrasis before this notion could be
eliminated.
Consider such a statement as, "Brutus killed Caesar." On another
occasion, Brutus and Caesar might engage our attention, but for the
present it is the killing that we have to study. We may say that to kill
a person is to cause his death intentionally. This means that desire for
a person's death causes a certain act, because it is believed that that
act will cause the person's death; or more accurately, the desire and
the belief jointly cause the act. Brutus desires that Caesar should be
dead, and believes that he will be dead if he is stabbed; Brutus
therefore stabs him, and the stab causes Caesar's death, as Brutus
expected it would. Every act which realises a purpose involves two
causal steps in this way: C is desired, and it is believed (truly if the
purpose is achieved) that B will cause C; the desire and the belief
together cause B, which in turn causes C. Thus we have first A, which is
a desire for C and a belief that B (an act) will cause C; then we have
B, the act caused by A, and believed to be a cause of C; then, if the
belief was correct, we have C, caused by B, and if the belief was
incorrect we have disappointment. Regarded purely scientifically, this
series A, B, C may equally well be considered in the inverse order, as
they would be at a coroner's inquest. But from the point of view of
Brutus, the desire, which comes at the beginning, is what makes the
whole series interesting. We feel that if his desires had been
different, the effects which he in fact produced would not have
occurred. This is true, and gives him a sense of power and freedom. It
is equally true that if the effects had not occurred, his desires would
have been different, since being what they were the effects did occur.
Thus the desires are determined by their consequences just as much as
the consequences by the desires; but as we cannot (in general) know in
advance the consequences of our desires without knowing our desires,
this form of inference is uninteresting as applied to our own acts,
though quite vital as applied to those of others.
A cause, considered scientifically, has none of that analogy with
volition which makes us imagine that the effect is _compelled_ by it. A
cause is an event or group of events, of some known general character,
and having a known relation to some other event, called the effect; the
relation being of such a kind that only one event, or at any rate only
one well-defined sort of event, can have the relation to a given cause.
It is customary only to give the name "effect" to an event which is
later than the cause, but there is no kind of reason for this
restriction. We shall do better to allow the effect to be before the
cause or simultaneous with it, because nothing of any scientific
importance depends upon its being after the cause.
If the inference from cause to effect is to be indubitable, it seems
that the cause can hardly stop short of the whole universe. So long as
anything is left out, something may be left out which alters the
expected result. But for practical and scientific purposes, phenomena
can be collected into groups which are causally self-contained, or
nearly so. In the common notion of causation, the cause is a single
event--we say the lightning causes the thunder, and so on. But it is
difficult to know what we mean by a single event; and it generally
appears that, in order to have anything approaching certainty concerning
the effect, it is necessary to include many more circumstances in the
cause than unscientific common sense would suppose. But often a probable
causal connection, where the cause is fairly simple, is of more
practical importance than a more indubitable connection in which the
cause is so complex as to be hard to ascertain.
To sum up: the strict, certain, universal law of causation which
philosophers advocate is an ideal, possibly true, but not _known_ to be
true in virtue of any available evidence. What is actually known, as a
matter of empirical science, is that certain constant relations are
observed to hold between the members of a group of events at certain
times, and that when such relations fail, as they sometimes do, it is
usually possible to discover a new, more constant relation by enlarging
the group. Any such constant relation between events of specified kinds
with given intervals of time between them is a "causal law." But all
causal laws are liable to exceptions, if the cause is less than the
whole state of the universe; we believe, on the basis of a good deal of
experience, that such exceptions can be dealt with by enlarging the
group we call the cause, but this belief, wherever it is still
unverified, ought not to be regarded as certain, but only as suggesting
a direction for further inquiry.
A very common causal group consists of volitions and the consequent
bodily acts, though exceptions arise (for example) through sudden
paralysis. Another very frequent connection (though here the exceptions
are much more numerous) is between a bodily act and the realisation of
the purpose which led to the act. These connections are patent, whereas
the causes of desires are more obscure. Thus it is natural to begin
causal series with desires, to suppose that all causes are analogous to
desires, and that desires themselves arise spontaneously. Such a view,
however, is not one which any serious psychologist would maintain. But
this brings us to the question of the application of our analysis of
cause to the problem of free will.
V. The problem of free will is so intimately bound up with the analysis
of causation that, old as it is, we need not despair of obtaining new
light on it by the help of new views on the notion of cause. The
free-will problem has, at one time or another, stirred men's passions
profoundly, and the fear that the will might not be free has been to
some men a source of great unhappiness. I believe that, under the
influence of a cool analysis, the doubtful questions involved will be
found to have no such emotional importance as is sometimes thought,
since the disagreeable consequences supposed to flow from a denial of
free will do not flow from this denial in any form in which there is
reason to make it. It is not, however, on this account chiefly that I
wish to discuss this problem, but rather because it affords a good
example of the clarifying effect of analysis and of the interminable
controversies which may result from its neglect.
Let us first try to discover what it is we really desire when we desire
free will. Some of our reasons for desiring free will are profound, some
trivial. To begin with the former: we do not wish to feel ourselves in
the hands of fate, so that, however much we may desire to will one
thing, we may nevertheless be compelled by an outside force to will
another. We do not wish to think that, however much we may desire to act
well, heredity and surroundings may force us into acting ill. We wish to
feel that, in cases of doubt, our choice is momentous and lies within
our power. Besides these desires, which are worthy of all respect, we
have, however, others not so respectable, which equally make us desire
free will. We do not like to think that other people, if they knew
enough, could predict our actions, though we know that we can often
predict those of other people, especially if they are elderly. Much as
we esteem the old gentleman who is our neighbour in the country, we know
that when grouse are mentioned he will tell the story of the grouse in
the gun-room. But we ourselves are not so mechanical: we never tell an
anecdote to the same person twice, or even once unless he is sure to
enjoy it; although we once met (say) Bismarck, we are quite capable of
hearing him mentioned without relating the occasion when we met him. In
this sense, everybody thinks that he himself has free will, though he
knows that no one else has. The desire for this kind of free will seems
to be no better than a form of vanity. I do not believe that this desire
can be gratified with any certainty; but the other, more respectable
desires are, I believe, not inconsistent with any tenable form of
determinism.
We have thus two questions to consider: (1) Are human actions
theoretically predictable from a sufficient number of antecedents? (2)
Are human actions subject to an external compulsion? The two questions,
as I shall try to show, are entirely distinct, and we may answer the
first in the affirmative without therefore being forced to give an
affirmative answer to the second.
(1) _Are human actions theoretically predictable from a sufficient
number of antecedents?_ Let us first endeavour to give precision to this
question. We may state the question thus: Is there some constant
relation between an act and a certain number of earlier events, such
that, when the earlier events are given, only one act, or at most only
acts with some well-marked character, can have this relation to the
earlier events? If this is the case, then, as soon as the earlier events
are known, it is theoretically possible to predict either the precise
act, or at least the character necessary to its fulfilling the constant
relation.