To understand Kant you have to, of course, understand what he is rejecting. If you don’t the know context of a philosopher, which is always what problem they are facing, then you won’t really be able to understand the point of their work. You will, for example, think it is easy to dismiss a whole or part of their argument, because it disagrees with some contemporary position (as though they were guilty of some unforeseen stupidity on their part, as though they should have none better). Thus Plato is dismissed because he thinks forms can be separate from instantiations of them, or Aristotle because he believed that reality was made of 5 elements, or Descartes because he believed in God, and so on. Of course, once one has grasped the context of a philosopher that does not mean that one has to take on board all the they say, but it does mean that one won’t dismiss them in a superficial way.
One way of understanding Kant’s philosophy is seeing how it arises out of the perennial conflict between empiricism and rationalism in Western Philosophy (though we shall see that it is not as simple as simply unifying them as some might believe). We have already spent some time in discussion of rationalism because of our lecture on Descartes, so this time we will look at, in a little detail, empiricism, and more specifically Hume.
He famously woke Kant from his ‘dogmatic slumbers’, since before reading Hume, he was a rationalist of a kind. What then is the basis of Hume’s philosophy? We do not require unjustified metaphysical speculation in order to have a rational scientific understanding of nature. To rid ourselves of this metaphysical speculation we have to become sceptical has to the objective basis of science, but this is necessary if we are not to base it on fictional and imaginary ideas. For Hume the source of all our ideas are the senses. This limitation is very important for Kant. Like Hume, he will argue that our knowledge of the world is limited to what is given in experience. Outside of that we can know nothing. In this sense, Kant is more Humean than he is Cartesian.
Sensations themselves are divided into two for Hume. On the one had there are impressions, and on the other ideas. Impression are direct sensations. I see the colour blue. Blue is the immediate sensation. Ideas are the relations between impression. I see many blue things, and I compare them through the concept ‘blue’. This does not mean ideas are separate from impression in terms of their existence. An idea exists only because there are impressions, and these impressions have their source in the sensation of the world. Ideas are, if you like, impressions that have become older. They are less vivid and present than immediate sensations, but they are made of nothing but sensations. A blind man, Hume argues, could not have an idea of a colour, because he has not seen it, nor a deaf man sound, because he has not heard it (Hume & Buckle 2007, p.16). Hume’s question is whether there is a necessary order in the relations of ideas, as there is in the order of impressions (in sensation, one impression comes after the other). In other words, what groups or orders my ideas together. If I think of x, must I also think of y?
The answer is that I associate one idea with another. There are three kinds of association for Hume, resemblance, proximity and causality. If I see a picture of a fox, then I am likely to think of a fox, if I imagine a room in the university, then I am likely to think of a room next to it, and finally, if I think of stone dropping from someone’s hand, then I likely to think of falling to the ground. Now it is the last association that is fundamental to how we think of the explanations of natural sciences. When we think of explanations in total, then are two kinds: relations of ideas and matters of fact. For the former, Hume is thinking of logic and mathematics. For these, we do not have to go beyond the ideas themselves (if you understand the meaning of one idea, then you will know why the other idea is necessarily associated with, so that ‘bachelor’ must mean ‘unmarried man [remembering that these ideas still have their origin in impressions]). But for matters of fact this is not the case, because they tell us something new about the world, rather than just analyse what we already know. Why do we believe that the sun will rise tomorrow, when we could equally believe the opposite. Hume is not arguing that we shouldn’t believe that the sun will rise (in fact he has good argument to think why we do), but there is no logical reason why we shouldn’t. The reason why we do is that we associate one idea with the other, the idea that the sun rose yesterday with the idea that the sun will rise tomorrow. We might think that we get to this second idea through an argument, where the statement ‘the sun rose yesterday’ is a premise. If it is an argument of this kind, then it could only be a relation of ideas or a matter of fact. It can’t be the first, since there is no contradiction in thinking the opposite, but it can’t be a matter of fact, because it is precisely that kind of argument I am trying to prove, so I appear to be going around in circles.
The answer must be that my conviction must have its origin elsewhere and that a belief is not the same a giving a reason or having a reason (indeed Hume will argue that our reasons have their source in our beliefs rather than the other way around). His answer is that the source of this belief is in our impressions rather than in our ideas first of all. It is because I have had the vivid experience of the sun rising again and again in the past. The belief that it will do so in the future is a habit and custom of the mind that I associated with the impression of I am having now. Thus when I see the see the dawn, whether directly or indirectly, I immediately associate it with the idea of the sun rising and I cannot help but do so because this custom or habit belongs to human nature intrinsically. A belief then is a particular vivid idea. Not as vivid as a direct sensation, but more vivid than a reason or a concept, and it is this that cause me always to associate x with y. Of course experience is open ended. It is perfectly possible that one day my belief will be unconfirmed rather than confirmed by experience.
Kant is more on Hume’s side, as we have said, rather than Descartes. In these sense, he is an empirical realist, that our understanding and experience of the world is given by experience, and we cannot deduce facts about the world by arguing from ideas. Where he differs from Hume is how far he is willing to take this. He argues that causality cannot be just an habit of mind, an association, but must be fundamental to our experience as such, so fundamental that we would even be having experiences of objects at all, rather than worrying whether the sun might rise tomorrow or not.
What is fundamental to Kant’s difference from both Descartes and Hume is how he conceives of the relation between the subject and object. For both of them, though they give completely diametrically opposed answers to the problem, it is a question of how the subject conforms to the object. For Descartes, my knowledge conforms to the object through ideas, whereas for Hume, it does so through sensations. For Kant, on the contrary, and it is this that is totally novel in his approach, the relation between the subject and object must be reversed. It is not how does the subject conform to the object, but how does the object conform to the subject. As Kant writes,
Hitherto it has been assumed that all our knowledge must conform to objects. But all our attempts to extend our knowledge of objects by establishing something in regard to them a priori, by means of concepts, have, on this assumption, ended in failure. We must therefore make trial whether we may not have more success in the tasks of metaphysics, if we suppose that objects must conform to our knowledge […] We should then be proceeding on the lines of Copernicus’ primary hypothesis. (Kant 2007, p.Bxvi)
Both Hume and Descartes relate knowledge to knowing the thing as it is in itself. One asserts that we can now it through ideas, the other through sensations. But it precisely this ‘thing’ that we cannot know, Kant argues. We can know how the object appears to us. Appearance itself is split into two: the content of appearance and the form of appearance. The content of appearance is what is given in experience (what Hume calls sensations or impressions). The form is how these contents appear to us. Thus, we can distinguish between what the chair is, and how the chair appears to us.
It is Kant’s argument that the form of appearance is universal and necessary. Unlike the habits of Hume, then, they are true of all human cognition, and we cannot experience the world in any other way. In the transcendental aesthetic, Kant describes the pure forms of sensation, time and space; in the transcendental logic, the pure forms of the object (the categories of the understanding); and finally in the transcendental dialectic, how philosophy gets into difficulties when it treats these pure forms as though they were objects of experience that one could know directly.
Let us look at Kant’s argument for the pure form of space in the Critique of Pure Reason, because by examining this one argument we will see how Kant’s employs a transcendental method to solve the age old antagonism between empiricism and rationalism. Kant is arguing that space and time are a priori and synthetic. What he means by that is that space is prior to experience but also adds something to experience (it unifies it; this is the formal element). We can already see that Kant is doing something novel here, because usually we think that the a priori is analytic, and the synthetic is a posteriori, so it seems quite strange to argue that space and time are a priori and synthetic.
Let us first of all look at the belief that space is something real, just like the objects that we can see. Kant’s argument against this common sense view is quite simple: space, he argues is the outer form of things for us. In other words, things that are outside of us are always in space (the difference from time, is that this is the inner form of ideas – are memories are not literally in space). To say that space is derived from experience is therefore to beg the question, for the very thing that one is trying to prove, spatiality, is already appealed to in the proof. Secondly, space cannot be derived from experience because of its necessity. The necessity of space for every appearance is that it is possible to imagine space without any appearances, no tree, no house and so, but it is not possible to imagine the absence of space and appearances (of course it is possible to think the absence of space, but it is not possible to imagine that there is something and no space). The argument against the Newtonian view that space is something real (a self-subsisting entity, as Kant calls it) is that this would mean that space were a container, but this container itself would have to contained and so on ad infinitum.
This would seem to imply that space, therefore, can only be a concept of things rather than something real, but Kant has to show that space is the pure form of sensations, and not just a concept that we have of things. This is much harder to prove than that space is not real and Kant, for this reason, spends more time doing so. The philosopher he has mind, who thinks space is just a relational concept, is Leibniz. For Leibniz, space is not something, so to speak that exists outside of objects and thus independently of them, rather it is an idea that expresses the relation between objects. Space is, therefore, not something real, but merely an idea. How can we best understand this notion of relational ideal space? Think of two places and the distance between them. Let us think of the two cities of Plymouth and Exeter in the South-West of England. We might say that Exeter is near to London than Plymouth, but is this being nearer a property of Exeter, or does it not rather express the relation between Exeter and Plymouth. For there to be space at all, there needs to be at least two objects. With just one object, there would be no space. If space were not a relation between objects, then it would exist, if there were only one object.
Kant needs to show that space is not simply a thought that we associate with objects, but their necessary form of presentation. He does this by showing that how we use pure intuition of space, is not the same as how we use concepts. The intuition of space is unitary, singular and unique. This means that diverse spaces are parts of one and the same Space. The relation between these spaces, and Space, is not the same as the relations between the concept Tree and instances of trees. All the diverse parts of space belong to one space, but trees do not belong to one and the same Tree, therefore space cannot be a concept, and if it cannot be concept it must be an intuition, since these are the only two sources of human knowledge.
So the conclusion of the argument is that space is not property of things, either as sensation or a concept, but is a necessary part of our experience of the world, for we cannot have an experience of an external without it already been organised through space. Space, therefore, is both a priori, since it is necessary, and synthetic, since it is added something to experience (namely it is spatial). Things are spatial because human consciousness is spatial, and not the other way around. Kant repeats the argument with time, and then in the transcendental logic with logics. It wants also to show through the transcendental deductions that without these pure forms of appearance there would be no object for us at all. As Kant writes,
The a priori conditions of a possible experience in general are at the same time conditions of the possibility of objects of experience. (Kant 2007, p.A111)
So Hume’s argument is that we have an experience and then associated these ideas in our minds. For Kant, on the contrary, we wouldn’t be having an experience at all. Causality is not something that we apply to our experience; it belongs to the very fabric of our experience as something meaningful and coherent from us, and it is on this foundations that the natural sciences are built.
Gardner, S., 2006. Kant and the Critique of pure reason, London; New York: Routledge.
Hume, D. & Buckle, S., 2007. An enquiry concerning human understanding and other writings, Cambridge; New York: Cambridge University Press.
Kant, I., 2007. Critique of Pure Reason 2nd ed., Palgrave Macmillan.
Kant, I. et al., 2004. Immanuel Kant Prolegomena to Any Future Metaphysics., Cambridge: Cambridge University Press.
 Of Leibniz-Wolffian kind. Kant writes in Prolegomena, ‘I freely admit that it was the remembrance of David Hume which, many years ago, first interrupted my dogmatic slumber and gave my investigations in the field of speculative philosophy a completely different direction’ (Kant et al. 2004, p.10).
 We need to be certain here, as Sebastian Gardner points out, that Kant is not denying that there is a concept of space, but we should not confuse this with the pure intuition of space, which must underlie even this concept (Gardner 2006, p.77).
 Kant, in the second edition of The Critique of Pure Reason, also demonstrates the difference between the pure intuition of space and a concept by demonstrating that the infinity of space is not the same as the infinity of a concept.