Immanuel Kant is generally regarded as the greatest of the modern
philosophers. Though he lived through the Seven Years War and the
French Revolution, he never interrupted his teaching of philosophy at
Königsberg in East Prussia. His most distinctive contribution was the
invention of what he called the 'critical' philosophy, which, assuming
as a datum that there is knowledge of various kinds, inquired how such
knowledge comes to be possible, and deduced, from the answer to this
inquiry, many metaphysical results as to the nature of the world.
Whether these results were valid may well be doubted. But Kant
undoubtedly deserves credit for two things: first, for having perceived
that we have _a priori_ knowledge which is not purely 'analytic', i.e.
such that the opposite would be self-contradictory, and secondly,
for having made evident the philosophical importance of the theory of
knowledge.
Before the time of Kant, it was generally held that whatever knowledge
was _a priori_ must be 'analytic'. What this word means will be best
illustrated by examples. If I say, 'A bald man is a man', 'A plane
figure is a figure', 'A bad poet is a poet', I make a purely analytic
judgement: the subject spoken about is given as having at least two
properties, of which one is singled out to be asserted of it. Such
propositions as the above are trivial, and would never be enunciated
in real life except by an orator preparing the way for a piece of
sophistry. They are called 'analytic' because the predicate is obtained
by merely analysing the subject. Before the time of Kant it was thought
that all judgements of which we could be certain _a priori_ were of this
kind: that in all of them there was a predicate which was only part
of the subject of which it was asserted. If this were so, we should be
involved in a definite contradiction if we attempted to deny anything
that could be known _a priori_. 'A bald man is not bald' would assert
and deny baldness of the same man, and would therefore contradict
itself. Thus according to the philosophers before Kant, the law of
contradiction, which asserts that nothing can at the same time have and
not have a certain property, sufficed to establish the truth of all _a
priori_ knowledge.
Hume (1711-76), who preceded Kant, accepting the usual view as to what
makes knowledge _a priori_, discovered that, in many cases which had
previously been supposed analytic, and notably in the case of cause and
effect, the connexion was really synthetic. Before Hume, rationalists at
least had supposed that the effect could be logically deduced from the
cause, if only we had sufficient knowledge. Hume argued--correctly, as
would now be generally admitted--that this could not be done. Hence he
inferred the far more doubtful proposition that nothing could be known
_a priori_ about the connexion of cause and effect. Kant, who had been
educated in the rationalist tradition, was much perturbed by Hume's
scepticism, and endeavoured to find an answer to it. He perceived that
not only the connexion of cause and effect, but all the propositions
of arithmetic and geometry, are 'synthetic', i.e. not analytic: in
all these propositions, no analysis of the subject will reveal the
predicate. His stock instance was the proposition 7 + 5 = 12. He pointed
out, quite truly, that 7 and 5 have to be put together to give 12: the
idea of 12 is not contained in them, nor even in the idea of adding them
together. Thus he was led to the conclusion that all pure mathematics,
though _a priori_, is synthetic; and this conclusion raised a new
problem of which he endeavoured to find the solution.
The question which Kant put at the beginning of his philosophy, namely
'How is pure mathematics possible?' is an interesting and difficult one,
to which every philosophy which is not purely sceptical must find
some answer. The answer of the pure empiricists, that our mathematical
knowledge is derived by induction from particular instances, we have
already seen to be inadequate, for two reasons: first, that the validity
of the inductive principle itself cannot be proved by induction;
secondly, that the general propositions of mathematics, such as 'two
and two always make four', can obviously be known with certainty by
consideration of a single instance, and gain nothing by enumeration of
other cases in which they have been found to be true. Thus our knowledge
of the general propositions of mathematics (and the same applies to
logic) must be accounted for otherwise than our (merely probable)
knowledge of empirical generalizations such as 'all men are mortal'.
The problem arises through the fact that such knowledge is general,
whereas all experience is particular. It seems strange that we should
apparently be able to know some truths in advance about particular
things of which we have as yet no experience; but it cannot easily be
doubted that logic and arithmetic will apply to such things. We do not
know who will be the inhabitants of London a hundred years hence; but
we know that any two of them and any other two of them will make four of
them. This apparent power of anticipating facts about things of which
we have no experience is certainly surprising. Kant's solution of the
problem, though not valid in my opinion, is interesting. It is, however,
very difficult, and is differently understood by different philosophers.
We can, therefore, only give the merest outline of it, and even that
will be thought misleading by many exponents of Kant's system.
What Kant maintained was that in all our experience there are two
elements to be distinguished, the one due to the object (i.e. to what we
have called the 'physical object'), the other due to our own nature. We
saw, in discussing matter and sense-data, that the physical object is
different from the associated sense-data, and that the sense-data are to
be regarded as resulting from an interaction between the physical
object and ourselves. So far, we are in agreement with Kant. But what
is distinctive of Kant is the way in which he apportions the shares of
ourselves and the physical object respectively. He considers that the
crude material given in sensation--the colour, hardness, etc.--is due
to the object, and that what we supply is the arrangement in space
and time, and all the relations between sense-data which result from
comparison or from considering one as the cause of the other or in any
other way. His chief reason in favour of this view is that we seem
to have _a priori_ knowledge as to space and time and causality and
comparison, but not as to the actual crude material of sensation. We can
be sure, he says, that anything we shall ever experience must show the
characteristics affirmed of it in our _a priori_ knowledge, because
these characteristics are due to our own nature, and therefore
nothing can ever come into our experience without acquiring these
characteristics.
The physical object, which he calls the 'thing in itself',(1) he regards
as essentially unknowable; what can be known is the object as we have it
in experience, which he calls the 'phenomenon'. The phenomenon, being
a joint product of us and the thing in itself, is sure to have those
characteristics which are due to us, and is therefore sure to conform
to our _a priori_ knowledge. Hence this knowledge, though true of all
actual and possible experience, must not be supposed to apply outside
experience. Thus in spite of the existence of _a priori_ knowledge, we
cannot know anything about the thing in itself or about what is not
an actual or possible object of experience. In this way he tries to
reconcile and harmonize the contentions of the rationalists with the
arguments of the empiricists.
(1) Kant's 'thing in itself' is identical in _definition_ with
the physical object, namely, it is the cause of sensations. In the
properties deduced from the definition it is not identical, since Kant
held (in spite of some inconsistency as regards cause) that we can know
that none of the categories are applicable to the 'thing in itself'.
Apart from minor grounds on which Kant's philosophy may be criticized,
there is one main objection which seems fatal to any attempt to deal
with the problem of _a priori_ knowledge by his method. The thing to
be accounted for is our certainty that the facts must always conform to
logic and arithmetic. To say that logic and arithmetic are contributed
by us does not account for this. Our nature is as much a fact of the
existing world as anything, and there can be no certainty that it will
remain constant. It might happen, if Kant is right, that to-morrow
our nature would so change as to make two and two become five. This
possibility seems never to have occurred to him, yet it is one which
utterly destroys the certainty and universality which he is anxious
to vindicate for arithmetical propositions. It is true that this
possibility, formally, is inconsistent with the Kantian view that time
itself is a form imposed by the subject upon phenomena, so that our
real Self is not in time and has no to-morrow. But he will still have
to suppose that the time-order of phenomena is determined by
characteristics of what is behind phenomena, and this suffices for the
substance of our argument.
Reflection, moreover, seems to make it clear that, if there is any truth
in our arithmetical beliefs, they must apply to things equally whether
we think of them or not. Two physical objects and two other physical
objects must make four physical objects, even if physical objects cannot
be experienced. To assert this is certainly within the scope of what
we mean when we state that two and two are four. Its truth is just as
indubitable as the truth of the assertion that two phenomena and two
other phenomena make four phenomena. Thus Kant's solution unduly limits
the scope of _a priori_ propositions, in addition to failing in the
attempt at explaining their certainty.
Apart from the special doctrines advocated by Kant, it is very common
among philosophers to regard what is _a priori_ as in some sense mental,
as concerned rather with the way we must think than with any fact of
the outer world. We noted in the preceding chapter the three principles
commonly called 'laws of thought'. The view which led to their being so
named is a natural one, but there are strong reasons for thinking
that it is erroneous. Let us take as an illustration the law of
contradiction. This is commonly stated in the form 'Nothing can both be
and not be', which is intended to express the fact that nothing can at
once have and not have a given quality. Thus, for example, if a tree
is a beech it cannot also be not a beech; if my table is rectangular it
cannot also be not rectangular, and so on.
Now what makes it natural to call this principle a law of _thought_
is that it is by thought rather than by outward observation that we
persuade ourselves of its necessary truth. When we have seen that a tree
is a beech, we do not need to look again in order to ascertain whether
it is also not a beech; thought alone makes us know that this is
impossible. But the conclusion that the law of contradiction is a law
of _thought_ is nevertheless erroneous. What we believe, when we believe
the law of contradiction, is not that the mind is so made that it must
believe the law of contradiction. _This_ belief is a subsequent result
of psychological reflection, which presupposes the belief in the law of
contradiction. The belief in the law of contradiction is a belief about
things, not only about thoughts. It is not, e.g., the belief that if we
_think_ a certain tree is a beech, we cannot at the same time _think_
that it is not a beech; it is the belief that if the tree _is_ a
beech, it cannot at the same time _be_ not a beech. Thus the law of
contradiction is about things, and not merely about thoughts; and
although belief in the law of contradiction is a thought, the law of
contradiction itself is not a thought, but a fact concerning the things
in the world. If this, which we believe when we believe the law of
contradiction, were not true of the things in the world, the fact
that we were compelled to _think_ it true would not save the law of
contradiction from being false; and this shows that the law is not a law
of _thought_.
A similar argument applies to any other _a priori_ judgement. When we
judge that two and two are four, we are not making a judgement about our
thoughts, but about all actual or possible couples. The fact that our
minds are so constituted as to believe that two and two are four, though
it is true, is emphatically not what we assert when we assert that two
and two are four. And no fact about the constitution of our minds could
make it _true_ that two and two are four. Thus our _a priori_ knowledge,
if it is not erroneous, is not merely knowledge about the constitution
of our minds, but is applicable to whatever the world may contain, both
what is mental and what is non-mental.
The fact seems to be that all our _a priori_ knowledge is concerned with
entities which do not, properly speaking, _exist_, either in the mental
or in the physical world. These entities are such as can be named by
parts of speech which are not substantives; they are such entities as
qualities and relations. Suppose, for instance, that I am in my room. I
exist, and my room exists; but does 'in' exist? Yet obviously the word
'in' has a meaning; it denotes a relation which holds between me and my
room. This relation is something, although we cannot say that it exists
_in the same sense_ in which I and my room exist. The relation 'in' is
something which we can think about and understand, for, if we could not
understand it, we could not understand the sentence 'I am in my room'.
Many philosophers, following Kant, have maintained that relations are
the work of the mind, that things in themselves have no relations,
but that the mind brings them together in one act of thought and thus
produces the relations which it judges them to have.
This view, however, seems open to objections similar to those which we
urged before against Kant. It seems plain that it is not thought which
produces the truth of the proposition 'I am in my room'. It may be true
that an earwig is in my room, even if neither I nor the earwig nor any
one else is aware of this truth; for this truth concerns only the earwig
and the room, and does not depend upon anything else. Thus relations, as
we shall see more fully in the next chapter, must be placed in a world
which is neither mental nor physical. This world is of great importance
to philosophy, and in particular to the problems of _a priori_
knowledge. In the next chapter we shall proceed to develop its nature
and its bearing upon the questions with which we have been dealing.