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I want to introduce Kant’s philosophical approach in the Critique of Pure Reason – also known as the First Critique – by looking at what he took to be Hume’s sceptical stance on causation, and how, in general terms, he sought to overcome it. When Kant himself set out the main threads of his argument in his own introductory essay on the First Critique, unappealingly entitled Prolegomena to Any Future Metaphysics that will be able to Present itself as a Science, it was his reaction to Hume’s scepticism about causation that he particularly singled out. He did so not only because Hume’s scepticism awoke him from his dogmatic slumbers, but, more crucially, because it gave him the hint of the correct approach to philosophical problems:

Since Locke’s and Leibniz’s Essays, or rather since the beginning of Metaphysics as far as the history of it reaches, no event has occurred which could have been more decisive in respect of the fate of this science than the attack that David Hume made on it. He brought no light into this kind of knowledge, but he struck a spark at which a light could well have been kindled, if it had found a receptive tinder and if the glow had been carefully kept up and increased. (Prol, Preface; 4:257).

Hume’s attack on causation was aimed at the principle that every event, or change of state, in nature must have a cause. He did not deny that we believed the principle to be true. What he denied was that we were justified in our belief. For the principle claims necessity as well as universality: it states that every event in nature must have a cause. How, asks Hume, could such a connection, a universal and necessary connection, possibly be proved? Not by experience; that is, not by perceiving how particular events in the spatio-temporal world behaved. For no amount of experience could prove that every event has a cause. The universal judgment is here taken to be entirely unrestricted, applying to all past, present and future events in nature, actual and possible. Evidently too, no experience could prove that it is necessary that any event has a cause. Experience can only tell us that such-and-such is or is not the case; it can never tells us that it must or must not be so.

But if experience will not do the trick, how could the causal principle be proved? The only alternative, Hume contended, is to show that it is true in virtue of the meaning of the terms involved. If the meaning of ‘event’ includes in it ‘having a cause’, then, indeed, we can justifiably assert that every event must have a cause. (Just as we can justifiably assert that every bachelor must be unmarried. In this latter case, the mere analysis of the subject term ‘bachelor’ and the predicate term ‘unmarried’ reveals that to deny the judgment would be self-contradictory.) But, as Hume argued, there simply is no such connection of meaning between the subject and the predicate terms in the principle ‘Every event must have a cause.’ To deny it is not self-contradictory. In Kantian terminology, the principle is not analytically true.

Since the principle is not analytically true, and, as Hume contended, this is the only acceptable way to prove that a judgment holds with strict universality and necessity, he concluded that our belief in the principle is unjustified.

Why, then, do we believe it? Here, Hume gives a psychological answer. It is the constant occurrence, throughout our past experience, of similar changes of state, under the same circumstances, that has led to our belief that the principle is justified. Far from the belief arising from, or being provable by, our rational faculties, it is merely the product of our enlivened imagination. In particular, the necessity that we ascribe to the principle is merely a ‘subjective necessity’ or feeling of inevitability (arising from our experience of past constant conjunctions), and not an objective necessity (not a requirement, discernible in the objects or in our judgment about the objects, that nature is uniform). Accordingly, so far as reason or understanding is concerned, our experience of nature could have been entirely chaotic. Moreover, there is absolutely no rational ground for supposing that our experience – even granting that it has, in fact, been as regular as clockwork up to now – might not turn random, acausal, at any moment in the future. The supposition that the future course of events will resemble the past cannot even be shown to be probable, let alone necessary.

It is important to grasp the extent to which Kant agreed with Hume’s position. First, he accepted that the causal principle cannot be proved by experience (since it claims necessity and universality). Second, he accepted that the necessity and universality attaching to the principle do not derive merely from the meaning of the terms involved. That is, he agreed with Hume that the principle is not analytically true. Third, he accepted that there is no way in which we could determine with certainty the truth of any specific causal claim in nature. That any particular kind of event actually occurs (e.g. that water in the liquid state does, under certain circumstances, turn to ice), and why it occurs (what its cause is), have to be left to experience to discover. We cannot prove that particular kinds of changes of states must occur, or what specifically their causes in nature must be.

But he disagreed with Hume about the status of the general principle that every event, or change of state, in nature must have a cause. Although he thought that Hume was correct to maintain that this principle is not analytically true, he rejected what he took to be Hume’s conclusion from this observation: namely, that the principle cannot be justified.

How, though, can the causal principle legitimately carry necessity and universality, if not in virtue of the meaning of the terms involved?

To understand Kant’s answer to this question is to be well on the way to understanding many of the central ideas in the Critique of Pure Reason. He was not exaggerating when he claimed that Hume had struck a spark which, if carefully kindled, would produce a new light on metaphysics. For Kant thought that the status of the causal principle could be generalized to take in not only all the leading judgments in metaphysics, but also all the fundamental judgments in two areas of what he saw as unquestionably genuine repositories of knowledge of objects: namely, pure mathematics and pure natural science (pure natural science forms the non-empirical basis of Newtonian physics). And this thought, in turn, led him to conclude that there must be something wrong with Hume’s scepticism. Since, as he affirmed, there certainly are two areas where we can find examples of judgments which, while not analytically true, hold with necessity and universality, viz. in pure mathematics and pure natural science, what is required is not a wholesale dismissal of all such knowledge claims, but an investigation of how such judgments can be true, in those two areas where they clearly exist.

In brief, Hume’s scepticism alerted Kant to the fact, or to what he took to be the fact, that lying at the basis of three central areas of knowledge or alleged knowledge of objects – mathematics and natural science, on the one hand, and metaphysics, on the other – are a host of judgments or principles of exactly the same status as the causal principle. As he saw it, the fundamental judgments in all three areas claim to hold with necessity and universality, and yet none of them can be proved in virtue of the meaning of the terms involved. Accordingly, if the only way of seeking to establish such a judgment were through an analysis of the terms involved, it would follow that none of these areas could contain informative instances of knowledge that hold with necessity and universality.At least in the cases of mathematics and natural science, Kant regarded this conclusion as absurd. Accordingly, he maintained that Hume must have been mistaken in dismissing the causal principle merely on the ground that, though the principle claims necessity and universality, it cannot be established in virtue of the meaning of the terms involved. The correct conclusion, Kant held, is that there must be some other way to establish (at least some) judgments of this kind. The strategy of the Critique of Pure Reason may essentially be seen as proceeding in two stages: in the first stage, it investigates how it is possible to establish these judgments in mathematics and natural science (where, as Kant sees it, they quite evidently exist); and on the basis of this investigation, it proceeds, in the second stage, to enquire whether the leading judgments of metaphysics can also be established.