Critique of Pure Reason

Critique of Pure Reason argues that the mind does not passively copy a ready-made world but actively constitutes experience. Space, time, and the categories such as causality and substance are not features of things-in-themselves but the necessary forms the mind imposes to make experience possible.

From this follows the central distinction between phenomena, the world as it appears and is structured by consciousness, and noumena, things as they are independent of all minds, which remain forever unknowable. Kant frames this reorientation as a Copernican revolution that places the knowing subject at the centre, so that the order found in nature is in part order the mind contributes.

It is arguably the single most important Western ancestor of the claim that consciousness constructs reality, cited wherever the observer is said to shape the observed. The honest framing is that Kant's idealism is transcendental and shared — the mind structures experience universally by fixed forms — and he denied that individual will could bend the empirical world.