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So far, we have concentrated on the first stage of Kant’s Copernican revolution: the investigation of how the judgments in pure mathematics and natural science can exist (as they actually do). But we saw that he also maintains that metaphysics is essentially made up of judgments which have the same status as those in mathematics and natural science. With metaphysics, however, it is by no means clear that its central claims can be known to be true: the protracted and indecisive debates about every one of them strongly suggests that they cannot. In the section of the First Critique entitled ‘Transcendental Dialectic’, he turns to the second stage of his Copernican revolution: the investigation of whether the central claims of metaphysics can be substantiated. He concludes that our theoretical reason is unable to show any of them to be true or false.

His ground for reaching this conclusion is closely connected with the first stage of his Copernican project. For Kant’s explanation of how judgments in pure mathematics and natural science can hold with necessity and universality, while yet not being analytically true, is that they make our experience, our empirical knowledge of spatio-temporal objects, possible. (I have tried to illustrate this with the particular case of the law of causality: this law is held to make possible our experience of change of states among spatio-temporal objects.) But the central (positive) claims of metaphysics – that the soul is immortal, that we possess free will, and that God exists – have no relationship to sense experience. They entirely transcend it: they can neither be shown to make sense experience possible nor, given their status, be confirmed or disconfirmed through sense experience. Since, as he argues, these are the only ways by which theoretical reason can establish any non-analytic judgment, he concludes that we cannot prove or disprove the central claims of metaphysics by theoretical means.

It is vital, however, to appreciate that Kant does not maintain that the impossibility of verification – either directly by sense experience or indirectly by showing that they make sense experience possible – renders the central claims of metaphysics to be effectively unthinkable. He allows that we can consistently think a judgment like the soul is immortal, even though we cannot confirm or disconfirm it by theoretical means. Certainly, a judgment can only be established – or even given any determinate meaning – by showing that it has a relation, direct or indirect, to experience. But Kant denies that it is necessary for thinking any of the propositions of metaphysics in an indeterminate way that we should be able to relate them to sense experience.

In actual fact (though here we need to go outside the First Critique), he holds that the central claims of metaphysics can be established.And he holds that they can be established by showing that they make experience possible. But, with the central claims of metaphysics, the experience is not sense experience, but moral experience, our recognition of duty and of the need to pursue the highest good. This recognition, however, is made known to us not by theoretical but by practical reason.

One way of understanding Kant’s moral philosophy is to see it as attempting to prove the key judgments of metaphysics by showing that they make our moral experience possible. His idea is that we can only explain the existence of our moral experience – the demands of which we cannot doubt – by acknowledging the truth of the central claims of metaphysics (just as, in the First Critique,he argues that we can only explain the existence of mathematics and natural science – the reality of which we cannot doubt – by acknowledging that space and time are properties of our mind, and that the fundamental laws of nature derive from concepts in our understanding).We shall obviously need to consider Kant’s proof of these metaphysical claims when we examine his Copernican revolution in relation to morality.

Returning to the Copernican revolution as this is exemplified in the First Critique, Kant’s main negative point is that the traditional methods of the metaphysician must be given up.The central claims of metaphysics cannot be established by employing theoretical reason: they are not true solely in virtue of the meaning of the terms involved, and it is useless to seek to employ mathematics or any of the principles of natural science outside possible sense experience. Any attempt to establish the key judgments of metaphysics by employing mathematics or natural science is bound, Kant argues, to be dialectical (i.e. to be fallacious). Yet with respect to theoretical reason, it is only the employment of mathematics and natural science that can possibly yield informative judgments having the same status as the central claims of metaphysics. Accordingly, so far as our search for knowledge is concerned, the message of the First Critique is that this search is defensible where it is based on sense experience, or where it bears upon the possibility of our having sense experience, but indefensible with respect to the central claims of metaphysics. In their case, we should admit our necessary ignorance, and renounce our quest for theoretical enlightenment. We should concentrate solely on the quest for knowledge in those areas that are related to sense experience. Here, indeed, we can establish the existence of informative necessary and universal principles or axioms, in addition to an unlimited amount of empirical, and so probable, knowledge.

The Dialectic is frequently represented in a wholly negative way: as Kant’s criticism of the use of theoretical reason outside the spheres of mathematics and natural science (as well as everyday sense experience). Certainly, the Dialectic does have this important negative side. But if we are to understand the place of the First Critique in Kant’s overall critical system, it is imperative also to grasp a more positive side to his attack on metaphysics, as traditionally conceived.

We saw that, in the course of this attack, he maintains that it must be impossible theoretically to prove or disprove freedom of the will, the immortality of the soul, or the existence of God. But this impossibility, it transpires, is fortunate: fortunate for morality quite as much as for our positive beliefs in freedom, immortality and God. Since, as he points out in the Preface to the second edition of the First Critique, if we do not embrace his Copernican revolution, we shall have to renounce these metaphysical beliefs; and this, in turn, will mean our acknowledging that the demands of morality are delusory (B xxvii–xxx).

But why does Kant suppose that his Copernican revolution is necessary for holding on to our central metaphysical beliefs? The answer is that he thinks that if we do not distinguish the world of our senses (the spatio-temporal world) from the world as it is in itself (the world that exists independently of our possible sense experience), then the deterministic laws that provably obtain in the sensible world must apply to us as moral agents. It would, in short, be impossible for us even to assume freedom of the will (see the Antithesis and Observation of the Third Antinomy). Equally, we should have to renounce our belief in a necessary Being who has created and sustains the universe; since, without the distinction, it is provably impossible that such a Being could exist (see the Antithesis and Observation of the Fourth Antinomy). Lastly, the belief that the soul is simple – and, therewith, the possibility of believing in the continuation of the soul after the death of the body – must be rejected, unless the world of the senses is distinguished from the world as it is in itself (see the Antithesis and Observation of the Second Antinomy).

Moreover, if it is impossible even to assume the existence of free will, God and the immortality of the soul, without embracing Kant’s Copernican revolution, then since – as he himself argues – these are necessary presuppositions of morality, it follows that the demands of morality must themselves be delusory.That is why Kant asserts:‘I have therefore found it necessary to deny knowledge in order to make room for faith’ (CPR,B xxx; italics original).

This famous assertion should not be taken merely as showing that, in order to save scientific knowledge, Kant accepted that we would have to deny ourselves any knowledge of the central claims of metaphysics (while leaving open the possibility of our believing in them). It implies something far stronger. It implies that if the traditional picture of our relationship with the world of the senses is correct, then we should actually be precluded from even believing in the existence of free will, God and immortality – since we would then be in possession of proofs of the impossibility of each of these beliefs. Only if we embrace Kant’s Copernican picture can we deny the force of the proofs, and thereby make room for the beliefs that are necessary for morality. So the metaphysical discussions in the Dialectic secure our belief in God, freedom and immortality – and thereby in morality also – against inevitable scepticism.

The First Critique, then, not only seeks to explain how there can be universal and necessary knowledge of objects in mathematics and in natural science, it also seeks to leave a space open for morality.As Kant sees it, neither the theoretical nor the practical side to our lives can be sustained on the traditional picture. On his Copernican picture, on the other hand, we can – in fact do – have both. The positive contribution of the Dialectic is to show how it is possible for the moral life to exist – and therewith to lay the ground for the practical proofs of just those central metaphysical claims (concerning freedom of the will, the immortality of the soul, and the existence of God) that, as he had argued earlier, our theoretical reason is, in reality, powerless to prove or disprove.