On the Introduction of Noumena
Kant introduces noumena in order to avoid the dialectical illusions, embodied by antinomies, in which reason would necessarily find itself when trying to cross the boundaries of experience, and into Metaphysics, trying to reach for the unconditioned.
Indeed, in the Preface to the Second Edition of the Critique of Pure Reason (all following are translations by J. M. D. Meiklejohn), he says:
Now, if it appears that when, on the one hand, we assume that our
cognition conforms to its objects as things in themselves, the
unconditioned cannot be thought without contradiction, and that when,
on the other hand, we assume that our representation of things as they
are given to us, does not conform to these things as they are in
themselves, but that these objects, as phenomena, conform to our mode
of representation, the contradiction disappears [...]
What he means by the unconditioned is the first term of any series of conditions e. g. the beginning of time. According to the arguments presented in the first antinomy, the beginning of time cannot be thought without contradiction, for:
- If time had a beginning, then it would have been preceded by nothing, and nothing could arise from nothing;
- If time didn't have a beginning, then an infinite amount of time would have actually occurred, which is impossible;
In the case that time is conceived as having a begin ning, it would be "too small" for reason, which always aims for the unconditioned. On the other hand, conceived as having no beginning, it is "too big" for the understanding, and cannot be conceived in accordance with experience. By distinguishing all things between noumena and phenomena, Kant aims to distinguinsh between what is purely thought in a thing and what is imagined in a thing.
Indeed, in Chapter III of the Analytic of Principles, he says:
[...] this conception [noumenon] is necessary to restrain sensuous
intuition within the bounds of phenomena, and thus to limit the
objective validity of sensuous cognition; for things in themselves,
which lie beyond its province, are called noumena for the very purpose
of indicating that this cognition does not extend its application to
all that the understanding thinks.
The unconditioned, on the other hand, is in itself a necessary concept of reason, which is the faculty by which the mind ascends the series of conditions, as if going up though the terms of syllogism. Reason is what, given some number of hypotheses, leads us to find an adequate thesis which would join these facts in a common knowledge. Because the unconditioned itself is necessary, it must be conceived in some way. But it can only be conceived in a negative way, as how things are as we can't know them: completely void of content.
Refutation of Idealism
Kant doesn't directly refer to "solipsism", but he presents an argument directed against Descartes' and Berkeley's doubts of the existence an external world under the name "Refutation of Idealism". Though he makes no reference to noumena nor to things in themselves, his argument lies in the critical assumption that time and the self are not in themselves perceptible, but are transcendental objects.
The argument is as follows:
The simple but empirically determined consciousness of my own
existence proves the existence of external objects in space.
I am conscious of my own existence as determined in time. All
determination in regard to time presupposes the existence of something
permanent in perception. But this permanent something cannot be
something in me, for the very reason that my existence in time is
itself determined by this permanent something. It follows that the
perception of this permanent existence is possible only through a
thing without me and not through the mere representation of a thing
without me. Consequently, the determination of my existence in time is
possible only through the existence of real things external to me.
Now, consciousness in time is necessarily connected with the
consciousness of the possibility of this determination in time. Hence
it follows that consciousness in time is necessarily connected also
with the existence of things without me, inasmuch as the existence of
these things is the condition of determination in time. That is to
say, the consciousness of my own existence is at the same time an
immediate consciousness of the existence of other things without me.