PROLEGOMENA TOANYFUTUREMETAPHYSICS 799
or not. But the difference between truth and dreaming is not ascertained by the nature of
the representations which are referred to objects (for they are the same in both cases), but
by their connection according to those rules which determine the coherence of the repre-
sentations in the concept of an object, and by ascertaining whether they can subsist
together in experience or not. And it is not the fault of the appearances if our cognition
takes illusion for truth, that is, if the intuition, by which an object is given us, is considered
a concept of the thing or even of its existence which the understanding can only think. The
senses represent to us the course of the planets as now progressive, now retrogressive; and
herein is neither falsehood nor truth, because as long as we hold this to be nothing but
appearance we do not judge of the objective character of their motion. But as a false judg-
ment may easily arise when the understanding is not on its guard against this subjective
mode of representation being considered objective, we say they appear to move back-
ward; it is not the senses however which must be charged with the illusion, but the under-
standing, whose province alone it is to make an objective judgment from appearances.
Thus, even if we did not at all reflect on the origin of our representations, whenever
we connect our intuitions of sense (whatever they may contain) in space and in time,
according to the rules of the coherence of all knowledge in experience, illusion or truth will
arise according as we are negligent or careful. It is merely a question of the use of sensuous
representations in the understanding, and not of their origin. In the same way, if I consider
all the representations of the senses, together with their form, space and time, to be nothing
but appearances, and space and time to be a mere form of the sensibility, which is not to be
met with in objects out of it, and if I make use of these representations in reference to pos-
sible experience only, there is nothing in my regarding them as appearances that can lead
astray or cause illusion. For all that they can correctly cohere according to rules of truth in
experience. Thus all the propositions of geometry hold good of space as well as of all the
objects of the senses, consequently, of all possible experience, whether I consider space as
a mere form of the sensibility or as something cleaving to the things themselves. In the
former case, however, I comprehend how I can know a priorithese propositions concern-
ing all the objects of external intuition. Otherwise, everything else as regards all possible
experience remains just as if I had not departed from the common view.
But if I venture to go beyond all possible experience with my concepts of space
and time, which I cannot refrain from doing if I proclaim them characters inherent in
things in themselves (for what should prevent me from letting them hold good of the
same things, even though my senses might be different, and unsuited to them?), then a
grave error may arise owing to an illusion, in which I proclaim to be universally valid
what is merely a subjective condition of the intuition of things and certain only for all
objects of senses—namely, for all possible experience; I would refer this condition to
things in themselves, and not limit it to conditions of experience.
My doctrine of the ideality of space and of time, therefore, far from reducing the
whole sensible world to mere illusion, is the only means of securing the application of one
of the most important kinds of knowledge (that which mathematics propounds a priori) to
actual objects and of preventing its being regarded as mere illusion. For without this obser-
vation it would be quite impossible to make out whether the intuitions of space and time,
which we borrow from no experience and which yet lie in our representation a priori,are
not mere phantasms of our brain to which objects do not correspond, at least not ade-
quately; and, consequently, whether we have been able to show its unquestionable validity
with regard to all the objects of the sensible world just because they are mere appearances.
Secondly, though these my principles make appearances of the representations
of the senses, they are so far from turning the truth of experience into mere illusion
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