We have already argued that one way of thinking about Kant’s thought is as the mediation of the two competing camps of modern philosophy, rationalism and empiricism. Such mediation, however, will entirely formal and empty, if we don’t understand the particular nature of Kant’s response to both camps; that is to say what he takes and rejects from rationalism, and what he takes and rejects from empiricism. Such a selection itself would not be possible if Kant were not able to conceive of a completely different way of thinking about subjectivity that he calls transcendental idealism. So it is not just a matter of joining two opposite philosophical schools together, which would be impossible and nonsensical, but of looking of the problem they intended to answer, in an entirely different way.
What was that problem? How do we join together or bridge the subjective with the objective world. In other words how do we know that what think, see, and feels is exactly the same as what is out there. How do we know that our representations are true at all? We shall take, for reasons of brevity, Descartes and Hume as our representatives for rationalism and empiricism respectively and we shall first of all begin with Descartes.
The most important thing to emphasis in reading Descartes, as Hatfield quite rightly emphasises in his introduction to the Meditations, is that we cannot make sense of his philosophical work without acknowledging their scientific origin.(Hatfield 2003, pp.1–36) Descartes’ problem, then, was how to justify philosophically the new science of the 17th century and to do so he had to reject the traditional Aristotelian metaphysics he was taught and which was the general way in which his contemporaries thought about nature. For this reason, we too must remind ourselves, if very schematically, what this metaphysics is.
At the heart of Aristotelian metaphysics is substance. Perhaps when we ordinarily think about this word we think of matter. When I ask you ‘what is the substance of this table?’, then you might answer ‘atoms’ or some such other word. But this is not how Aristotle thought about substance (in fact he had an entirely different word for matter, which was hyle). Aristotle’s word for substance is ousisa, which means ‘being’ or ‘a being’. In other words ‘substance’ names everything that is. The question then when I ask you ‘what is the substance of this table?’, is what makes this thing a table as opposed to chair, or horse and so on. It can’t be matter, because matter doesn’t tell me why such and such a thing is what it is opposed to something else. What is, on the contrary, are individual things, such as tables, chairs and horses, and this is what Aristotle calls substances. Indeed, as he says, it is about such things that we make judgements; the table is brown, the chair is round, the horse is lame, and so on. Substance, therefore are the subjects of propositions. What something is made up of is its matter and form. The form of something tells is why it is what it is, whereas matter is only what is formed.
Now, of course, there is a lot more detail in Aristotle’s conception of substance than this, but for our purposes this is all that we need to know at the moment. It is precisely this view, however, that is shattered by the new science. In the Aristotelian conception of the universe, since it is made up of individual substances, every substance has to have its own explanation, which makes his science extremely complex and unwieldy. Also, many of the individual predications of this science where shown through observation to be false. For example, Aristotle placed the earth at the centre of the universe and divided it into realms the mundane and the extra-mundane (basically everything on earth, and everything above the earth). The extra-mundane realm was immutable and perfect (not subject to corruption and change as in the mundane realm). This meant that everything in the universe was meant to rotate around the earth. Yet Galileo’s observation of the moons of Jupiter showed that there were rotations independent of the earth. Rather than observation describing a multi-differentiated universe, it demonstrated that that the earth was not at the centre, that there numerous planets, and that the universe followed the same laws that were few in number (three according to Descartes).
Nature, therefore, was not made of infinitely many individual substances, but the one substance that Descartes postulated was made for corpuscles of matter that were infinitely divisible and whose movement and motion could be described accurately using mathematics. Thus any phenomenon we saw, such as the light of the rainbow, could be described in its own terms, as the movement and motion of such corpuscles.
At least in his physics, Descartes did not claim that this view was certain; that is to say, he put it forward as an hypothesis that worked. His defence of the new science was therefore practical. If we accept this view of nature, then we can see that we can solve the scientific problems that remained unanswerable, or had become impossibly complex in the old Aristotelian science. What he wished to achieve in his metaphysical writing, such as the Discourses and the Meditations, was a philosophical defence of this physics. How do we know (especially since the corpuscular view of nature is not something that I see through direct observation but is only a hypothesis) that the view of nature is true, that things really are what we say they are. In other words, that there really is a bridge from the subjective to the objective world?
This is where Descartes’ view of substance becomes more complex. For in fact there is not one substance but three: God, Matter and Mind. The physical hypothesis of corpuscular nature is first of all an idea and a mathematical one. Descartes’ scepticism, which everyone knows about is, is to find those ideas that we cannot doubt, but also more importantly, to separate our minds from our senses. For his argument is that we don’t understand nature first of all through our sense and then construct an idea, but our ideas is how we make sense of our senses. The idea is first. If the idea is first, then its legitimacy cannot be guaranteed by the senses that it explains. The first stage of Descartes’ method is to make us doubt the truth of our senses. This is a traditional part of philosophical scepticism and we can read the same arguments in Plato. How can I trust my senses when I know that they lie to me? How do I know that the whole of reality is not a dream? But the second stage of doubt is more interesting. For even Plato did not doubt that mathematical truths were an accurate representation of external things. But Descartes asks, how do I know that even the truths that I am certain of are really certain. For couldn’t a malicious demon simply put in my head that 2 + 2 = 4, when in fact it is 5? All I can be certain of is that if I am thinking something that it is I who am thinking it, for even I doubt that 2 + 2 = 4, then I cannot doubt that I am doubting it. Thus, as long as I remain within my inner space of my consciousness then I know what I am thinking is what I am thinking. When I think a chair I know that I am thinking a chair and so on. What I cannot know, however, is whether what I think is what the chair is in reality, or that what I think is the same as what you think. Thus we seemed trapped inside ourselves unable to get out each of us closed in our own worlds.
How does Descartes spring the trap? The answer is the malicious demon. He has to prove that this is only a thought and not a possible reality at all, and in fact the opposite is the case. This is because there is one thought which must have a reality that corresponds to it and that is the thought of God (which is the exact opposite of the malicious demon). I have the idea of God in my mind, but I cannot be the origin of that thought since I am a finite being and this is an infinite thought (not just in terms of its content, but what the thought is, its objective reality). The only origin of this thought must be God Himself who actually exists. We now see that a gap is opened up in ourselves to the outside (the outside in this case being God). This God, who is perfect and infinite and the highest reality), would not have created a universe in which there was no agreement between the material world and the inner truth of my consciousness. Thus Descartes’ argument is that the 3 laws that he discovered in his Physics are the continuous creations of God, and that God put the understanding of this laws into his mind, such that the one agrees with the other.
It might not surprise you that not every philosopher accepted this argument an one such was Hume. We might characterise Hume’s position as we don’t need to rid ourselves of scepticism in order to have a rational scientific understanding of nature and that Descartes went too far in his metaphysical speculations in order to do so. For Hume the source of all our ideas is perceptions. It is important to underline here that he doesn’t ask why that is so. He believes that it is impossible to for us to answer this question. It is precisely because Descartes thinks he can that the ends up with his unfounded metaphysical speculation. This limitation is very important for Kant, who is more Humean than he is Cartesian in this regard, and this is why he writes a critique of reason, which means nothing less than what are the necessary limits of reason, one of which would be that the only source of knowledge that we have is impressions. Perceptions themselves are divided into two: impressions and ideas. Impressions are what we might call sensations, like the colour blue that I am seeing now. Ideas are the concepts and thoughts that we have of this impressions. Ideas are only different in kind from impressions. An idea, if you like is an older impression, one that is less vivid and present than an impression. Thus, Hume would, say a blind man cannot have an idea of the colour blue because he has never seen such a colour. Simple ideas have their origin directly in impressions, but complex ones do not, because I can associated ideas in my mind without directly being present to the object, or even the object existing at all (think of the idea of the unicorn, which is made up the impression of two other objects, which are not present, the horse and horn). Hume’s question is whether there is a necessary order in my ideas as there is in my impressions (one impression comes after another one). In other words what is it that orders or groups my thought together. Why when I think of x do I also think of y?
The answer is that I associate the one with the other and there are three such principles of association: 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 principle, cause and effect, which Hume thinks is central to how we think about human knowledge or understand of nature. The understanding, Hume, says is divided into two: relations of ideas and matters of fact. In the former, he is thinking of such things like logic and mathematics. For the first we do not have to go beyond the operation of ideas themselves (if you understand the one idea, then you understand why necessarily the other idea is associated with it, since if you understand bachelor you will understand why every unmarried man is one). But in ideas that are matters of fact this is not the case. 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 disconfirmed rather than confirmed by experience.
I would say that Kant is more on Hume side than he is Descartes. In other words he takes it as given that our experience of the world is real. That we really do have sensations and that these sensations are external and not merely the product of minds. In other words, he is an empirical realist. Where he differs from Hume is that he is not convinced that causality is merely a habit of the mind. Of course this difference rest upon how one takes this ‘merely’. How necessary is a habit or custom? One might read Hume to say that it is pretty necessary. However such a necessity is only empirical and descriptive, which is how Hume limits his investigations, whereas for Kant causality is constitutive. In other words there wouldn’t be any experience whatsoever if we didn’t have this category. In other words, for Kant, if is not that we have experience x, and then experience y and then subsequently associate them in our minds through the principle of cause and effect, but that we would have any experience of x or y without it, even though, as he agrees with Hume, causality itself is not part of our actual experience. In Kant’s language, causality is a priori and synthetic. A priori, because it is not itself experiential, and synthetic, because it is actually doing some work, organising and making sense of our experience. The question, then, is what is the difference between the a priori synthetic, and a ‘custom and habit of the mind’. It is only by reading the Critique of Pure Reason that we shall discover it.
Hatfield, G., 2003. Routledge philosophy guidebook to Descartes and the meditations, London: Routledge.
 The three laws of bodily motion: What is in motion always continues to move; always moves in a straight line; a colliding body whose motion is greater will cause the motion to deflect, or if less carries that body with it. These three laws form the basis of Newton’s laws of motion.