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