Immanuel Kant is widely acknowledged by philosophers of all
persuasions to have been one of the greatest thinkers of all
time. He is also notorious for being one of the most difficult to
understand. The complexity of his prose, however, is not due to
any wilful obscurantism; in reading Kant, one is aware of a thinker
struggling to clothe in language ideas of the very highest level of
complexity and profundity.
Born in 1724, Kant lived his entire life in the East Prussian
town of Konigsberg. He never married, though he was a popular man
who by all accounts led a life of the utmost order and
regularity. His unique contribution to Western thought is his
'Critical Philosophy': a devastating and ingenious critique of
both speculative rationalistic metaphysics, and unfettered
empiricism. And yet this monumental system of thought, as set out
in the Critique of Pure Reason, stems from just one seemingly
humble question: how are synthetic a priori truths possible?
Kant introduced the distinction between 'analytic' and
'synthetic' judgements, despite its having been implicit in the
works of Hume and Aristotle, amongst others. He characterises an
analytic judgement as one in which 'the predicate B belongs to the
subject A, as something which is (covertly) contained in this
concept A'. (Critique of Pure Reason) The favourite example
of philosophers is 'All bachelors are unmarried'. Here, the
predicate ('are unmarried') simply 'unpacks' the conceptual content
of the subject ('bachelors'). A distinguishing feature of such
propositions is that they tell us nothing about the way the world
is; instead, they simply clarify what is involved in our concepts.
In the case of synthetic judgements, by contrast, Kant tells us
that the predicate 'lies outside the subject
concept'. (ibid.) An example might be 'All humans are under
twenty feet tall'. Whilst this proposition is no doubt true, it is
nonetheless certainly not a feature of the concept 'human' that
anything falling under it is under twenty feet tall. Thus, 'All humans
are under twenty feet tall' gives us a substantial piece of
information about the world, rather than about the concepts we apply
to that world.
It should be easy to see that analytic truths are a
priori: that is, knowable independently of any particular
experience. I do not have to carry out a survey of bachelors to
find out that they are all unmarried. But how could any
synthetic truth - one which gives us real information about
the world - be a priori?
Kant was of course aware that the vast majority of synthetic
truths are knowable only a posteriori - that they require
verification through experience. 'All humans are under twenty feet
tall' could never be known a priori. He held, however, that there
exists a special class of propositions that are both informative
and knowable independently of this or that experience. The
truths of mathematics (perhaps most significantly those of
geometry), he maintained, fall into this class, as do certain other
propositions, such as 'Every event has a cause'.
There is nothing about the concept of 7+5 that dictates that it
should be equal to 12, nor about the concept of a straight line
that it should be the shortest distance between two points. And yet
the propositions '7+5=12' and 'A straight line is the shortest
distance between two points' are both knowable a
priori. Similarly, it is not part of the concept of an event
that it should have a cause, and yet we can know with absolute
certainty, thinks Kant, that any event will be caused. But how can we
know such truths a priori?
Kant's answer to this question is both radical and
astonishing. Let us start with the case of geometry. There can only,
thinks Kant, be one explanation of our a priori knowledge of
the properties of space: the spatial properties of the world
must be contributed by the knowing subject. That is, the world
as it is in itself is not made up of objects arranged in
space. Only the world as it appears to us is spatial, and
this is precisely because space is nothing more than our way of
representing the world to ourselves. In Kant's own terminology,
space is nothing more than a form of intuition (that is, a form of
sensory perception). Kant employs a similar, though much weaker,
argument to conclude that time, too, is a mere form of
intuition. Space and time are features of the phenomenal world - the
world as it appears to us - only. The noumenal world - the world of
things as they are in themselves - is aspatial and atemporal.
Similarly, causal relations have a subjective origin, being, as
it were, 'projected' into the world by the experiencing
consciousness. Consequently, causation too is a feature only of
the world of appearances, and not of the world independent of our
cognitive faculties. However, whereas the forms of intuition are
features of our faculty of sensibility (the passive faculty that
receives sense impressions), causation is one of twelve
'categories', or 'a priori concepts' imposed on sense
impressions by the understanding (the active faculty of reason).
Kant's epistemology stands as a critique of both empiricism and
rationalism. The empiricist view is wrong, since the mind is not a
mere tabula rasa which passively receives knowledge of the
world through the senses. The rationalist view is just as mistaken,
as reason alone can never give rise to knowledge, since knowledge
demands both concepts and the raw data supplied by the
senses. Thus speculative metaphysics - the attempt to achieve
theoretical knowledge of non-empirical subjects such as the
existence of God, freedom, and immortality - inevitably falls
into illusion. It aims to gain knowledge of the world as it is in
itself, but theoretical knowledge can only be of the world as it
appears.
However, whilst Kant held that we have no theoretical
knowledge of such things, he maintained that we can have a
practical knowledge of them. Consider free will. When I
think of my actions as constituents of the phenomenal world, I am
obliged to regard them as produced by rigid deterministic laws, but
when I consider those same actions as they are in the noumenal world,
I am not so obliged. I can have practical knowledge of that
freedom which I am required to postulate in order to account for
my inescapable sense of myself as a responsible moral agent.
It seems to many that a choice has to be made between two apparently
incompatible ways of looking at the world: the spiritual and the
ethical on the one hand, and the scientific on the other. If Kant
is right, the dichotomy between these two ways of looking at the
world is purely illusory. There is room in the world for both
determinism and freedom, for science and spirituality.