The conditions of experience › Appearances and things considered in themselves

Kant calls his position transcendental idealism. The forms through which objects are given and thought constrain our knowledge of them as appearances. That does not turn an appearance into a hallucination. A desk can be an empirically real external object while our cognition of it remains subject to human conditions. The distinction is between an object as cognized under those conditions and a thing considered independently of them.

The vocabulary of noumena needs care. Kant distinguishes a negative sense, a thing considered apart from our sensible mode of intuition, from a positive sense, an object of intellectual intuition. He denies that humans possess such intellectual intuition. The negative notion marks a limit; it does not open a second territory in which we can describe otherwise hidden objects. Simply replacing "things in themselves" with "noumena" can conceal those distinctions. Analytic of Principles, phenomena and noumena.

Kant also argues against treating external experience as an optional inference from a self-sufficient inner world. In the Refutation of Idealism, he argues that consciousness of one's existence as determined in time depends on something permanent in perception external to oneself. The argument is a defense of external experience, not an inference to a detailed description of things in themselves. It also does not establish that every apparent perception is veridical: particular experiences still need examination. Refutation of Idealism.