By 'concepts' Kant meant merely making inferences based on logic without any grounding to the human sensible element at all.Kant in CPR wrote:[Bxvi] 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.
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.
This approach has ended in terrible failures and Kant listed all the serious failures in the CPR.
E.g. based on the Ontological, Cosmological and Philosophical Theological argument theists could argue there is a God, i.e. there are creations [common sense and scientifically justified], so there must be a creator, given the complexity there must a a supreme creator.
But to date there is no convincing proof God exists are real except theists are only clinging to faith.
Since the conceptual [no sensible elements] approach has failed miserably, Kant proposed we make the attempt to justify what is the really-real object by grounding and conforming the object on the human constitution or conditions.
In the above, Kant was stating if we focus on the objects as things-in-themselves, it is impossible to know them as things-in-themselves.[Bxvii]We should then be proceeding precisely on the lines of Copernicus' primary Hypothesis
If Intuition [of Objects] must conform to the constitution of the Objects [as Things-in-themselves], I do not see how we could know anything of the latter [the Objects as Things-in-Themselves] 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. [as Object of the Senses]
By the end of the CPR, Kant had demonstrated the things-in-themselves which are absolute mind-independent are illusory.
Kant then proposed his Copernican Revolution, i.e. instead of a focus of things as thing-in-themselves existing mind-independently out there, we should rather, focus on things are conditioned upon the human faculty of intuition [the human sensible faculty]. In this case, it would be possible to conceive the reality of things within the empirical world.
(this approach is vulnerable to the possible hallucinations and other weaknesses, but Kant eliminate those weaknesses philosophical critical thinking).
The point is the whole she-bang of serious philosophical problems arise from the philosophical realists ideological grasp of absolute mind-independent things, i.e. things-in-themselves exist regardless of whether there are humans or not.
As Kant alluded [B397], this is driven by some primal psychology where the ignorant humans will keep reifying and chasing the illusory things-in-themselves as absolutely real based on crude, raw under-developed Pure Reason, thus his Critique of Pure Reason.
Kant on the other hand suspend judgment on the above as absolutely mind-independent but rather posit mind-independence as relative only and contingent upon the human conditions [sensible faculty with the intellectual faculty].
Upon demonstrating things-in-themselves or the thing-in-itself as illusion[s], Kant nevertheless acknowledge that such illusions are useful for his moral theory.