Kant's 'Copernican Revolution'

The Copernican Turn

In a famous passage in the second edition of the Critique of Pure Reason Kant makes an analogy between the strategy pursued by his critical philosophy and the central contribution of Nicolas Copernicus.

Up to now it has been assumed that all our cognition must conform to the objects; but all attempts to find out something about them a priori through concepts that would extend our cognition have, on this presupposition, come to nothing. Hence let us once try whether we do not get farther with the problems of metaphysics by assuming that the objects must conform to our cognition, which would agree better with the requested possibility of an a priori cognition of them, which is to establish something about objects before they are given to us. This would be just like the first thoughts of Copernicus, who, when he did not make good progress in the explanation of the celestial motions if he assumed that the entire celestial host revolves around the observer, tried to see if he might not have greater success if he made the observer revolve and left the stars at rest. Now in metaphysics we can try in a similar way regarding the intuition of objects. (CPR Preface, Bxvi-xvii)

So just as Copernicus sought to explain the apparent motions of objects in the heavens in terms of the movement of the earthbound observer, so too Kant attempts to account for the apparent characteristics of objects in terms of our cognitive faculties and the cognitive conditions under which we know the objective world. In Kantian phrase, instead of assuming that our knowledge of the object must conform to it, we assume that it conforms to our knowledge.

Transcendental Idealism

Kant’s name for the position he articulates according to which objects must conform to our way of knowing them is ‘Transcendental Idealism.’ Though the exact meaning of Transcendental Idealism is much disputed by Kant’s interpreters, it is clear that he intends at least two things by it.

First, according to Transcendental Idealism, space and time are neither independent subsisting entities (as was suggested by Newton), nor object-dependent orders of relations between entities (as was argued by Leibniz). Instead, they are mind-dependent ‘forms of intuition.’ They are thus understood in terms of the characteristic ways in which we experience things rather than either being mind-independent things that we experience, or relations between mind-independent things that we experience. Kant argues that space and time are nevertheless ‘empirically real’ — they are fundamental features of the empirical world that we experience. But they are not real ‘in themselves.’ They have no ultimate reality apart from our capacity for experience.

Second, in addition to the transcendental ideality of space and time, to which all experienced objects must conform, Kant argues that empirical reality is itself further structured by a privileged set of a priori concepts.

experience itself is a kind of cognition requiring the understanding, whose rule I have to presuppose in myself before any object is given to me, hence a priori, which rule is expressed in concepts a priori, to which all objects of experience must therefore necessarily conform, and with which they must agree. As for objects insofar as they are thought merely through reason, and necessarily at that, but that (at least as reason thinks them) cannot be given in experience at all - the attempt to think them (for they must be capable of being thought) will provide a splendid touchstone of what we assume as the altered method of our way of thinking, namely that we can cognize of things a priori only what we ourselves have put into them (CPR Preface, Bxvii-xviii)

Kant here argues that the object which appear to one in perceptual experience, or ‘intuition’ as he calls it, also must conform to our basic conceptual scheme. He explains what he means here in the last sentence: objects must conform to our conceptual scheme because that scheme makes possible the experience of such objects.

Though elements of Kant’s ‘Copernican’ strategy remain unclear, the basic idea is that Kant argues that what makes synthetic a priori knowledge possible is the structure of our cognitive faculties, including the pure forms of intuition and a privileged set of a priori concepts. These forms and concepts jointly make experience possible and allow us to draw inferences that hold with necessity and universality concerning objects encountered in experience. What’s more, Kant argues that because a priori concepts are necessary for experience in general, we can know that those concepts are applied legitimately, since he is taking it as obvious that we have experience.


Allison, Henry E. 2004. Kant’s Transcendental Idealism: Revised and Enlarged. New Haven: Yale University Press.

Gardner, Sebastian. 1999. Kant and the Critique of Pure Reason. London: Routledge.

Guyer, Paul. 2014. Kant. London: Routledge.

Hogan, Desmond. 2010. “Kant’s Copernican Turn and the Rationalist Tradition.” In The Cambridge Companion to Kant’s Critique of Pure Reason, edited by Paul Guyer, 21–40. Cambridge: Cambridge University Press.

Van Cleve, James. 1999. Problems from Kant. Oxford: Oxford University Press.