In metaphysics we have to retrace our path countless times, because we fi nd that it does not lead where we want to go, and it is so far from reaching unanimity in the assertions of its adherents that it is rather a battlefi eld, and indeed one that appears to be especially determined for testing one’s powers in mock combat; on this battlefi eld no combatant has ever gained the least bit of ground, nor has any been able to base any lasting possession on his victory.
(Kant 1998: Bxv)
Kant opens his Critique of Pure Reason with a damning assessment of the state of metaphysics, especially when compared with the progress made in mathematics and science. He thinks that philosophers have attempted to have knowledge of reality through reason alone, but they have not succeeded: the problems they discuss, such as whether we have freedom of the will, are as controversial as they were in ancient Greece. Part of the problem, he thinks, is that a lot of the questions with which philos-ophers are concerned are in fact impossible for humans to answer, and part of the problem is that there is no established method for making progress in metaphysics. The point of his Critique is to solve these problems, by taking a step back and asking how it is possible for us to have substantial (nontrivial) knowledge of the world through reason alone, i.e., a priori. Kant thinks that an explanation of the possibility of metaphysical knowledge will give us a method for establishing metaphysical claims, at the same time as clearly delimiting which kinds of metaphysical questions it is possible for us to answer, thereby ending pointless dispute about those of which we cannot have knowledge.
Kant presents the Critique as addressed to the question, “How are synthetic a priori judgements possible?”, which he takes to be the same as the question, “How is metaphysics possible?” His two-part answer to the question of synthetic a priori knowledge is his account of how metaphysics is possible, as well as of the possible extent of metaphysical knowledge. Kant says that the question of how synthetic a priori judgements are possible is not a problem in logic, and he thinks that the only way of answering it is by invoking a complex and subtle metaphysical and epistemological
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position, which he calls “transcendental idealism.” This solution will explain what metaphysical knowledge Kant thinks is possible for us, as well as the principled limits he places on it. Further, Kant thinks that it provides the basis of empirical knowledge and science, at the same time as creating essential space for the quite different knowledge involved in thinking about morality.
Synthetic a priori knowledge
Kant says that all propositions are either analytic or synthetic, and are known either a priori or a posteriori. A priori knowledge is justifi ed independently of experience, and Kant thinks that all claims which contain necessity and universality can be known only a priori (Kant: A1). For example, the claim “every event has a cause” could never be established empirically, since we cannot, in principle, experience every event. For Kant, analytic propositions can be seen to be true through analysis (i.e., decomposition) of the concepts they contain (breaking them down into their sub-concepts [A151/B190]).
He also says that they can be seen to be true in accordance with the principle of non-contradiction (A6–7/B10–11); this brings his defi nition close to a common contemporary defi nition, which says that a proposition is analytic if it is a basic propo-sition of logic, or if it is translatable into a basic propopropo-sition of logic by substituting synonyms for synonyms. It is thus relatively easy to see how analytic propositions can be known a priori: through our grasping relations between the concepts in them. For example, the claim “bachelors are unmarried” can be analysed or decomposed into the claim “unmarried men are unmarried”; no investigation of particular bachelors is required to see that this is true. In contrast, synthetic propositions go beyond what is contained in the concept of the subject, and can be denied without contradiction; an example is the claim “bachelors are lazy.” This claim is synthetic and a posteriori: we need to investigate bachelors to see whether it is true. Synthetic a priori propositions will be more than mere logical claims, yet they cannot be seen to be true through inves-tigation of the way the world is, since they are a priori. For example, take the claim that
“every event has a necessitating cause.” Kant agrees with Hume both that this claim is not a truth of logic (it can be denied without contradiction) and that it cannot be established empirically. However, he does not agree with Hume that we should therefore dismiss it: Hume overlooks the possibility of synthetic a priori knowledge.
At the start of the Critique, Kant treats the possibility of our knowing metaphysical claims as an open question, but he thinks it is obvious that there is at least some synthetic a priori knowledge, since he thinks that mathematics is synthetic and a priori, and that physics is based on synthetic a priori claims. This view of mathematics is a reasonable one for Kant to hold, since it is not possible to reduce mathematics to logic given the logical tools available to him; indeed, whether this is completely possible with the logic we have today remains an open question. In the fi rst half of the twentieth century Kant’s claim that there is synthetic a priori knowledge was widely rejected, by those philosophers who went back to the empiricist idea that our a priori knowledge is restricted to analytic propositions. In the second half of the twentieth century, Kant’s account was rejected even more radically, by those who questioned the analytic–
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synthetic distinction, and the notion of a priority. There has not, however, been consensus around these rejections, and we will not concern ourselves with them here.
Why does Kant align the possibility of metaphysics with the question of synthetic a priori knowledge? Kant presents his position as a synthesis of rationalism and empir-icism. On the one hand, philosophers like Leibniz and Descartes think we can have substantial knowledge of the world through reason alone, independent of experience.
Kant agrees that this is what we aspire to in metaphysics, and he thinks that the drive to this kind of knowledge is a natural and unavoidable part of the way we think. But he thinks that all that these philosophers achieve is completely different accounts, neither supported nor contradicted by experience, and therefore with respect to which we have no clear way of adjudicating between them and of making progress. On the other hand, empiricists like Hume think that, in Kant’s terms, all our a priori knowledge is trivial, analytic knowledge, and that all our substantial knowledge is empirical. Hume ends his Enquiry Concerning Human Understanding by saying the following:
When we run over libraries, persuaded of these principles, what havoc must we make? If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the fl ames: for it can contain nothing but sophistry and illusion. (Enquiry, in Hume 1988: §12, pt 3)
If we accept this, then it seems that all we have are the empirical sciences on the one hand, and logic on the other, with nothing left for philosophy in between (the two main conceptions of philosophy in the twentieth century – as conceptual analysis, and as part of the natural sciences – seem to be a result of accepting this view). Against this view, Kant wants to fi nd a substantive role for philosophy, a metaphysics that is more than just clarifying relations between concepts used by the sciences. If metaphysics, as an investigation into reality, is to be different from the empirical sciences, and in particular, if we strive for knowledge of necessary truths, then metaphysics is a priori; for it to be substantive, it must be synthetic. Not only does Kant think that our minds strive after such knowledge, he thinks that empirical knowledge cannot be explained without it. While he is extremely impressed by the explanatory power and progress of the empirical sciences, there are two respects in which he thinks that they are limited.
First, he thinks we cannot explain empirical knowledge without appealing to some substantive a priori propositions which provide a framework within which empirical knowledge can proceed: we cannot gain knowledge from experience without principles with which to interpret the sensory input. Second, he thinks that there are limits to empirical knowledge, and there is reason to think that it cannot explain everything there is. Kant thinks that it is the failure to recognise the possibility of synthetic a priori knowledge that led Hume to dismiss metaphysics; at the same time, Kant respects the objections empiricists have to the idea that we could have substantial knowledge of mind-independent reality. Synthetic a priori judgments are the only way we could have
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non-trivial knowledge of reality through reason alone, yet synthetic a priori judgments seem to be a mystery (Kant: A10/B13).
Transcendental idealism
Kant’s answer is that our synthetic a priori knowledge is not knowledge of an entirely mind-independent world. In other words, he invokes a kind of idealism, his famous transcendental idealism. Kant’s transcendental idealism makes three basic claims. (1) There is a distinction between the world as it is in itself and the world as it appears to us. (2) The world as it appears to us depends on our minds, in some sense and to some extent. (3) We cannot have any knowledge of the world as it is in itself. Kant argues that our synthetic a priori knowledge is knowledge of the world as it appears to us, and that the fact that synthetic a priori truths hold of the world of experience is explained by the fact that our minds impose them on the world. As opposed to the tabula rasa of the empiricists, Kant thinks that the mind actively contributes to the way we experience the world as being, and that the world we experience is a combination of what is contributed by mind-independent reality and the structuring principles our minds use to process and arrange this input. Principles we use to interpret and structure our experience could not all have been derived from experience; some must be independent of it, and are therefore due to the mind and not the way the mind-independent world is. Thus, we can have nontrivial knowledge of necessary features of the world that we experience because the necessary structure of the world of experience is imposed on it by our minds. Kant famously says that “We can cognize of things a priori only what we ourselves have put into them” (Kant: Bxviii). What the mind contributes is a priori structure, which includes space and time, as well as a priori concepts and principles.
Ever since the publication of the fi rst edition of the Critique, and continuing to the present day, there has been no agreement amongst commentators as to how to interpret Kant’s transcendental idealism. Commentators disagree as to how Kant’s distinction between things in themselves and things as they appear to us should be understood, about the sense in which things as they appear to us depend on our minds, and about whether Kant’s idea of things as they are in themselves involves a metaphysical commitment to an actually existing but essentially unknowable aspect of reality, or whether he merely thinks that we cannot avoid using the concept of things as they are in themselves. Some commentators have read Kant as a strong metaphysical idealist in the way that Bishop Berkeley is – as thinking that empirical objects exist only in our minds. At the other extreme, Kant’s transcendental distinction between things as they are in themselves and things as they appear to us has been read as a merely epistemo-logical or methodoepistemo-logical distinction between two ways of thinking about the objects of knowledge, two aspects of the world, or two perspectives on the world. In between this defl ationary view and the extreme idealist view are any number of different accounts; what follows is one interpretation.
Repeatedly, and throughout the Critique, Kant makes strongly idealist-sounding claims, such as that
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if we remove our own subject or even only the subjective constitution of the senses in general, then all constitution, all relations of objects in space and time, indeed space and time themselves would disappear … as appearances they cannot exist in themselves, but only in us. (A42/B60)
It is hard to reconcile these statements with any interpretation of transcendental idealism that does not see Kant as committed to the world as we experience it being mind-dependent in a substantial sense. On the other hand, there are clear objections to seeing Kant as a Berkeleyan idealist with respect to appearances, not least that Kant himself explicitly and repeatedly rejects this interpretation of his position (B70, B274;
Prolegomena, in Kant 2004: 293, 374). Further, Kant explicitly rejects an important assumption that empiricists like Hume and rationalists like Descartes share, which is that what the mind is immediately and directly in contact with is something mental; on the contrary, he argues that we have immediate experience of objects distinct from us and in space (which he explicitly says are the very objects whose existence Berkeley denies and Descartes doubts, and these are clearly not mental entities), and that our knowledge of our mental life depends on this. As well as saying that the world as it appears to us depends on our minds, Kant also stresses what he calls its “empirical reality.” He says that appearances are transcendentally ideal but empirically real, and part of his concern in stressing their empirical reality is to distinguish appearances from mere mental states. Kant thinks that he can prove that empirically real objects are public, external objects, which exist in space and time, are made up of indestructible stuff (matter which is conserved), exist unperceived, and are in necessary causal relations with each other. One of the challenges of interpreting transcendental idealism is to do justice to both the empirical reality and the transcendental ideality of appearances.
Here is one way of thinking about the mind-dependence of appearances. Think of viewing a scene through a pane of glass. You do not see objects in virtue of seeing images on the pane of glass, rather, you see straight through the glass to the objects themselves. Now imagine that the glass has a distorting effect, such that, for example, the shapes of things are seen as being somewhat different to the way they actually are – perhaps as being more curved. Just as in the fi rst case, you do not see the objects in virtue of seeing images on the pane of glass; you still see through the glass to the objects beyond it. However, you now see the objects as being different, to some extent, from the way they are in themselves – the way they are independent of your perceiving them.
It would not be incoherent to say that what you see is in some sense a representation of the objects: what you see is the world as it is represented in your perceptual experience, which is partly dependent on its being seen through glass. At the same time you still see things which exist outside of your mind, although you do not see them as they are in themselves. Now imagine that the glass is more radically distorting, such that it actually affects how you see objects arranged in the scene – and even what you perceive as an object – and imagine that you have no way of fi nding out how objects are independ-ently of the way you perceive them. Put the cognitive-processing apparatus of the mind in the place of the glass, and you might think of Kant’s view as something like this. We are directly consciously presented with the world, as opposed to representing the world
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by constructing mental images, but the way we perceive things as being is partly a function of the way the mind is. Kant thinks that the way we perceive things is radically determined by our minds in such a way that the way they appear to us is nothing at all like the way they are independently of our perceiving them.
It is sometimes objected that Kant is not entitled to say both that there are things in themselves and that we cannot have any knowledge of them. However, what he says is that we cannot have any substantial knowledge of their nature, not that we cannot know any truths about them at all – for example, we can know that analytic proposi-tions are true of things in themselves. Kant thinks that we cannot have any nontrivial knowledge of the world as it is in itself: our empirical knowledge is limited to the world as it appears to us (which is partly dependent on our minds), and our a priori knowledge is limited to knowledge of the conditions of the possibility of experience (necessary truths about the world as it appears to us) and logic.
Kant’s distinction between things in themselves and things as they appear to us is almost, but not quite, the same as his distinction between noumena and phenomena;
the reason it is not quite the same is that Kant distinguishes between a positive and a negative notion of noumena. He says that a noumenon in the positive sense would be an object of a kind which, in principle, could not be experienced through our sense organs (think of Platonic numbers, God or Cartesian souls). Kant says that we do not even really understand what such objects would be (Kant 1998: B307), but that we have the concept of such objects as a limiting concept: it stops us from arrogantly asserting that there could not be more to reality than the kinds of things of which we have experience. But he says that a noumenon in the negative sense is a thing which we experience, thought of in abstraction from the ways we experience it (B307), and that this is not a merely limiting concept, but something to whose existence we are actually committed. This suggests that his notion of things in themselves is neither the notion of distinct supersensible objects, nor merely a limiting concept: rather, Kant thinks that there is an actually existing aspect of the world which is partly responsible for how the world appears to us, but of which we cannot have knowledge.
the reason it is not quite the same is that Kant distinguishes between a positive and a negative notion of noumena. He says that a noumenon in the positive sense would be an object of a kind which, in principle, could not be experienced through our sense organs (think of Platonic numbers, God or Cartesian souls). Kant says that we do not even really understand what such objects would be (Kant 1998: B307), but that we have the concept of such objects as a limiting concept: it stops us from arrogantly asserting that there could not be more to reality than the kinds of things of which we have experience. But he says that a noumenon in the negative sense is a thing which we experience, thought of in abstraction from the ways we experience it (B307), and that this is not a merely limiting concept, but something to whose existence we are actually committed. This suggests that his notion of things in themselves is neither the notion of distinct supersensible objects, nor merely a limiting concept: rather, Kant thinks that there is an actually existing aspect of the world which is partly responsible for how the world appears to us, but of which we cannot have knowledge.