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Unformatted text preview: riori synthetic knowledge that is, at least, unchallenged; we don’t have to ask
whether such knowledge is possible (for it is real), but only how it is possible. When we
can answer that, we shall know how to go about showing the possibility of all other kinds
of synthetic a priori knowledge.
Section 5: The general problem: how can there be knowledge based on pure reason?
We have seen the vast difference between analytic and synthetic judgments. It is easy to
see how there can be
they come purely from the law of contradiction. There is also no special problem about
how there can be
synthetic propositions that are known a posteriori,
that is, known from experience: experience itself is nothing but a continual joining
together of perceptions, ·so it is not surprising that it enables us to join concepts in a
synthetic way. Returning to an example used earlier, the synthetic proposition that some
bodies are heavy can be established through experiences in which Ÿperceptions of body
are joined with Ÿperceptions of weight·. What we do have a problem about is the
synthetic propositions that are known a priori.
Whatever makes this sort of knowledge possible, it is not the law of contradiction ·and it
is not experience·, so we must search to find out what it is.
But we cannot rightly start by asking whether synthetic a priori propositions are
possible. For there are plenty of them, really given to us with undisputed certainty; and as
our present procedure involves starting with what we already know, we shall start from
the premise that there is human a priori knowledge of some synthetic propositions. But
then we still have to ask how this knowledge is possible, that is, what makes it possible.
When we know that, we can learn how to use such knowledge and can learn what its
limits are. Stated precisely, then, the crucial question is this:
How is it possible to have a priori knowledge of synthetic propositions?
I have sometimes expressed this as a question about knowledge ‘out of pure reason’. That
is just another way of asking the same thing. When I speak here of knowledge out of pure 16
reason, I always mean knowledge of synthetic propositions, never of analytic ones; ·and of
course knowledge through pure reason is always a priori·. [At this point Kant has a
footnote commenting on the shift from the old senses of ‘analytic’ and ‘synthetic’
(explained on page 7 above) to his new senses for those terms.]
Metaphysics stands or falls with the solution to this problem. Someone may propound
his metaphysical claims as plausibly as he likes, smothering us with conclusions piled on
conclusions; but if he hasn’t first answered this question properly, we are entitled to say to
him: ‘This is all pointless ungrounded philosophy and false wisdom. You purport to be
using pure reason to create a priori knowledge, not by merely analysing concepts but by
making new connections that don’t rest on the law of contradiction; and you think you
have insight into these connections independently of all experience. But how do you get
such insight? How can you justify your claims?’ He cannot answer by appealing to the
common sense of mankind, for that is not evidence - it is mere hearsay. . . .
The question must be answered, but that is difficult to do. One reason why an answer
wasn’t attempted long ago is that a satisfactory answer to this one question demands
much deeper, more persistent and more careful thought than goes into the most lengthy
and ambitious metaphysical works ever published. (A weightier reason is that nobody
thought to ask the question!) Every reader who looks hard at the problem will initially be
frightened by its difficulty. Indeed, if it were not that there really is synthetic a priori
knowledge, the thoughtful person would think such knowledge to be impossible. This is
what happened to David Hume, although he didn’t put the question to himself in this
general form (which is the form we need if we are to get an answer that is decisive for the
whole of metaphysics). Hume asked an intelligent question: How can I arrive at a
judgment in which one concept is connected necessarily with another, even though the
one does not contain the other? He thought it couldn’t be done; which led him to conclude
that only experience can provide us with such connections. In other words, he thought
that this supposed necessity (which is the same as this supposed a priori knowledge) is
merely a long-standing habit o f accepting something as true, and hence of taking Ÿa
necessity in our thought - ·a mere mental compulsion· - to be Ÿa necessity in the world.
If you want to complain about the toil and trouble that I am going to give you in
solving this problem, I invite you to try solving it in an easier way! Perhaps that will make
you grateful to the man who has taken this deep task over for you, and you may even
come to be surprised - given how difficult the problem is - that the solution is not even
harder than it is. I have had to work for many years to solve this problem in...
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