"The only addition, properly so called– and that only in the method of proof– which I have made in the present edition, consists of a new refutation of psychological idealism, and a strict demonstration– the only one possible, as I believe– of the objective reality of external intuition. However harmless idealism may be considered– although in reality it is not so– in regard to the essential ends of metaphysics, it must still remain a scandal to philosophy and to the general human reason to be obliged to assume, as an article of mere belief, the existence of things external to ourselves (from which, yet, we derive the whole material of cognition for the internal sense), and not to be able to oppose a satisfactory proof to any one who may call it in question. ("from introduction to 'The Critique of Pure Reason')
To do this he has to admit to an internal dualism in which a permanent self is a subject that is conscious of its representations. Something has to be there to log the changes and to keep an eye on the shop. The objection that arises from this the classical one. You are only immediately aware of what is presented to your mind. That there is a corresponding external reality is an inference based on animal faith. Kant rejects this. I am aware of the reality of the changes in my self over time. It is a real intuition as real as I am myself and is therefore a type of inner representation that is connected to a reality.
"This consciousness of my existence in time is, therefore, identical with ,the consciousness of a relation to something external to me, and it is, therefore, experience, not fiction, sense, not imagination, which inseparably connects the external with my internal sense." (from introduction to 'The Critique of Pure Reason")
To which I might demur and in doing so hobble my reading but I will resist that and enjoy the monumental ambition of a thinker who is regarded as the most important of the early modern philosophers according to a poll top of the philosophers
Kant 421/Hume:232 Kant443 / Descartes 201 (ranked)
So numero uno but here's the thing: I have a suspicion (a hedged conviction) that of all those that voted a good number have never read 'The Critique of Pure Reason' through. As diligent students it would not have benefited them and when safeky graduated seem too historical. I spent a few weeks on it in a state of genial befuddlement.