How Kant Differs from Indirect Realism

Immediate Perception of Objects: Indirect realism claims that we are directly aware only of mental intermediaries (such as "sense data" or inner ideas) and must infer the existence of the external world. Kant explicitly rejects this. [1, 2, 3, 4]
Empirical Realism: Kant maintained an empirical realism, meaning we directly experience real, spatial, external objects in space and time—not mental pictures of them. In his Refutation of Idealism, he argued that outer experience is immediate and does not require an inference from internal states. [1, 2]
Transcendental Idealism: While external objects are real in the empirical world, Kant argued that space, time, and the categories of understanding are structures imposed by the human mind. Therefore, we do not know things as they are in themselves (Ding an sich), only as they appear to us (phenomena). [1, 2, 3]

Why the Label Sometimes Comes Up

Things-in-Themselves: Some critics and later philosophers (such as Schopenhauer) argued that Kant's separation between the unknowable thing-in-itself and the appearance in the mind leaves a vestige of pre-critical indirect realism.
Mediated Access to the Noumenal: While empirical perception is direct, our access to the underlying reality (noumena or things-in-themselves) is cut off, which can superficially resemble the separation found in indirect realist theories. [1, 2, 3, 4]