Kant had argued the b]noumenon[/b] aka thing-in-itself exists as a real thing[s] objectively within an independent external world.
Example among the typical;
viewtopic.php?p=508005#p508005
To convince philosophical realists otherwise is too tedious given that Kant's CPR is intrinsically complex and difficult to understand.
However a clue to the overriding theme that Kant believed there is NO ultimate independent external world is from his Copernican Revolution which goes like this;
- Hitherto it has been assumed that all our Knowledge must conform to Objects.
But all attempts to extend our Knowledge of Objects by establishing something in regard to them a priori, by means of Concepts, have, on this assumption, ended in Failure.
CPR Preface Bxvi
But all attempts to nail what is the real external object existing independent of human conditions has ended in failure.
Even Science would never succeed, leaning towards infinite regression, i.e. what is the ultimate things after quarks, and so on.
In philosophy note Substance Theory and all its criticisms.
https://en.wikipedia.org/wiki/Substance_theory
Kant in a way, 'mocked' the realists who, based on ignorance of his complete view, had the arrogance to insist Kant was following Berkeley's idealism;
- However harmless Idealism may be considered in respect of the essential aims of Metaphysics (though, in fact, it is not thus harmless),
it still remains a scandal to Philosophy and to Human Reason-in-General that the Existence of Things outside us (from which we derive the whole material of Knowledge, even for our Inner Sense) must be accepted merely on Faith,
and that if anyone thinks good to doubt their Existence, we are unable to counter his doubts by any satisfactory proof. B55
Kant challenged the realist that they cannot prove their philosophical realism is true.
So Kant suggested the alternative,
- We must therefore make trial whether we may not have more success in the tasks of Metaphysics, if we suppose that Objects must conform to our Knowledge.
This would agree better with what is desired, namely, that it should be Possible to have Knowledge of Objects a priori, determining something in regard to them prior to their being Given.
We should then be proceeding precisely on the lines of Copernicus' primary Hypothesis.
Failing of satisfactory progress of explaining the movements of the heavenly bodies on the supposition that they all revolved round the spectator, he tried whether he might not have better success if he made the spectator to revolve and the stars to remain at rest.
ibid
- A similar experiment can be tried in Metaphysics, as regards the Intuition of Objects.
If Intuition must conform to the constitution of the Objects, I do not see how we could know anything of the latter [the objects] a priori
but if the Object (as Object of the Senses) must conform to the constitution of our Faculty of Intuition, I have no difficulty in conceiving such a possibility.
CPR Preface Bxvii
Kant only mentioned sensibility in the above, but throughout the CPR the Understanding is also involved used in tandem with his Critical Philosophy.
The current position of the philosophical realist is they merely ASSUMED there is a real physical object beyond what what they can only be acquainted with their sense data.
Then they use the BOTTOM-UP approach with all sort of concepts, language, science in an attempt to prove and validate the ASSUMED real object is really real.
But because since humans can ONLY be acquainted with the sense-data of the assumed object, humans will never be able to realize what the assumed object really is.
As Russell doubted, 'perhaps there is no [real] table at all'.
https://en.wikisource.org/wiki/The_Prob ... /Chapter_1
In Kant's Copernican Revolution, he used the TOP-DOWN approach.
This meant Kant starts from what is Given empirically via experience and the human cognitive faculties [human conditions] and made the attempt to understand what is the ultimate real object.
Since Kant started with the human conditions, ultimately whatever the conclusion, it is conditioned upon the human conditions.
Kant's final conclusion is whatever that is ASSUMED by the philosophical realists to be ultimately real and is external and independent of the human conditions is an illusion when reified.
Since reality is ultimately conditioned upon the human conditions, philosophical realism, i.e. reality is independent of the human conditions cannot be true. Idealism [especially Kant Transcendental Idealism] is more realistic.