Immanuel Kant, answered from the texts and cited to the page.
The question concerns what I called, by analogy, a Copernican revolution in philosophy — and the analogy is precise. Copernicus, finding that the hypothesis of a sun revolving around a stationary earth produced nothing but confusion, reversed the assumption: let the earth move, and the appearances are explained. I performed the same inversion with respect to knowledge.
The dogmatic tradition had assumed that our cognition must conform to objects — that the mind, as it were, reaches out and takes the impress of things as they are in themselves. That assumption, pursued rigorously, leads straight into the skeptical impasse Hume identified: we cannot explain how necessary and universal judgments about nature are possible if all our knowledge is derived from experience of particular things.
My reversal: objects must conform to the conditions of our cognition, not the reverse. Space and time are not properties inhering in things as they are in themselves; they are forms of our sensibility, the manner in which the mind is constituted to receive appearances.1 Because any object that can appear to us must appear under these conditions, geometry and arithmetic — which articulate the structure of space and time — possess objective validity for everything we can ever encounter in experience, without being derived from experience.2
The Newtonian view that space and time are real beings existing independently of objects and observers makes this unintelligible: geometry would then be merely empirical, valid only so far as experience confirmed it, never necessarily so.3 The same inversion applies to the pure concepts of the understanding — causality, substance, and the rest.
Here the difficulty is sharper, because it is not immediately obvious why phenomena should conform to these concepts at all.4 Intuition can proceed without the understanding; objects can be given to sensibility without the categories being applied to them.5 The Transcendental Deduction — the most laborious section of the first Critique — establishes that any experience capable of being my experience, unified in one consciousness, must stand under the categories; it is not that the categories describe things as they are, but that the categories are the conditions under which any coherent experience is possible for a rational subject.
What the Copernican turn does not mean is that objects are therefore mere illusions, or that the mind fabricates reality from nothing. When I say that bodies appear to us in space and time as they affect our senses, I am not saying that bodies are mere illusory appearances.6 The object as phenomenon is genuinely given; it is distinguished from the object as thing-in-itself, not dissolved into a fiction.
Berkeley made the error of treating the ideality of space and time as grounds for degrading bodies to mere appearance; I argued explicitly that this error follows only if one ascribes objective reality to space and time as properties of things in themselves — precisely the move the critical philosophy refuses.7 The thing-in-itself remains as the limit-concept: there is a noumenal substrate that we cannot know theoretically, and that remainder is what prevents transcendental idealism from collapsing into the empirical idealism I spent the 1787 Refutation of Idealism repudiating.
the form of this intuition, which lies in the original constitution of the mind, determines, in the representation of time, the manner in which the manifold representations are to combine themselves in the mindCritique of Pure Reason, p. 24
space and time are pure intuitions, which contain à priori the condition of the possibility of objects as phenomena, and an à priori synthesis in these intuitions possesses objective validityCritique of Pure Reason, p. 48
The Newtonian view, that space and time are 'real beings' existing independently of objects, events, and observers, Kant argued, makes unintelligible how geometry (the science of space) can be known a priori to be valid for everything in space and time.Correspondence, p. 37
it hence remains doubtful à priori, whether such a conception be not quite void and without any corresponding object among phenomenaCritique of Pure Reason, p. 48
Phenomena would nevertheless continue to present objects to our intuition; for mere intuition does not in any respect stand in need of the functions of thought.Critique of Pure Reason, p. 48
When we say that the intuition of external objects, and also the self-intuition of the subject, represent both, objects and subject, in space and time, as they affect our senses, that is, as they appear—this is by no means equivalent to asserting that these objects are mere illusory appearances.Critique of Pure Reason, p. 24
if we regard space and time as properties, which must be found in objects as things in themselves...we cannot blame the good Berkeley for degrading bodies to mere illusory appearancesCritique of Pure Reason, p. 24