One of the most famous sentences in the Critique of Pure Reason is: ‘Thoughts without content are empty, intuitions without concepts are blind.’ (A51/B75) If we have a correct and thorough understanding of this sentences, we are already well on our way understanding Kant’s philosophical project. But to get there, we must delve a lot deeper than the usual popular interpretation. Here is what we might be commonly told:

In this passage, Kant is criticising, first, rationalism, and second, empiricism. According to Kant, all knowledge requires both input from sensory experience and the application of concepts. The problem with empiricism is that it assumes that we can know things without using concepts; while the problem of rationalism is that it assumes we can know things without using sensory experience. Thus, Kant sets out to move beyond both these traditions.

The problem with this interpretation is that it fails to explain the real import and depth of Kant’s criticisms. Moreover, it fails as a criticism of both empiricism and rationalism. After all, the empiricists do not believe that we can do without concepts — Locke, Berkeley and Hume all discuss concepts and famously disagree about how we should understand them. They would all agree that you can’t know that ‘some cows are black and white’ without having the concepts of cow, of black, and of white. And the rationalists do not believe that we humans can have much knowledge of the world without sensory input. None of them believes that rational thinking alone will teach you about the colour of cows.

So, what does Kant actually mean? The first thing we must realise is that Kant is not talking about the possibility of knowledge here, but more fundamentally about the possibility of cognition — that is, of object-related thought. What Kant is saying is that without both intuitions and concepts, we cannot even have thought about objects, thoughts that are made true or false by the way the world is. So if Kant is criticising empiricism and rationalism — and he is! — what he is saying is that both of these traditions make it impossible to understand how thinking about the world is so much as possible. Both of them are fundamentally mistaken about the nature of objective judgement.

Let’s start with empiricism. The crucial commitment of the empiricist tradition is that the mind works on the inputs of sensation in what is almost a mechanical fashion. This can be seen in the empiricist theory of concepts. The details differ, but the basic story is always one of association: we see many cows, the mind associates all these experiences with each other, and this generates the concept of a cow. This is, in effect, a psychological process. The aim of the philosopher is to explain how the mind works, how it associates things, generates concepts, and applies those concepts in new circumstances. This is essentially a descriptive task. There is no question of correctness here; we just describe what happens.

But this means, Kant would say, that thoughts cannot be wrong; not because they are always right, but because they cannot be right either. The mind is simply carrying out certain given operations; it is not acting under a norm. This means that the products of the operations cannot be right or wrong. Thoughts are like rocks. There are processes in the world that generate rocks; but since those processes are simply causal operations, not actions under a norm, it makes no sense to ask of a rock whether it is a good or a bad rock.

Now of course I can apply an external standard to rocks. Maybe I’m looking for a nice round and smooth rock to fashion into an ornament. And with this standard in mind I walk around and say: “This is a bad rock. This is bad rock. This rock is even worse. Oh, wow, this is a perfect rock!” But this is not a standard that is binding on rocks; it’s a standard that is binding on my activity of ornamentation. In the same way, we might be able to apply some external standard to thought. But it would not be binding on thought; it would not make anything a good or bad thought. Such a standard would not be a standard of rationality. Furthermore, it would be a standard that we couldn’t apply — because we cannot step outside of thought and then judge it, since judging is precisely thought.

This is very important. Many philosophers have thought that there is a standard of thought outside of thought: correspondence of thought to reality. The idea might be something like this. Our minds work in some way or other, which can be described as a causal, non-normative process; but we can nevertheless say that our minds are working well or badly; because the standard to which the mind is held is correspondence with reality. If the causal pathways of the mind generate thoughts that correspond to reality, the thoughts are true and the mind is working well. If the causal pathways of the mind generate thoughts that do not correspond to reality, the thoughts are false and the mind is working badly.

Kant does not deny that a true thought corresponds to reality. (Very few people deny this.) He does deny, although I will not go deeply into it here, that a purely causal item could stand in the relevant relation of correspondence. (Consider this: does a rock correspond to reality? The question makes no sense. But it would make no more sense to ask this of a causally generated thought. It corresponds to reality in the sense that it was what got created when reality impinged on the causal structures of the mind. What more could be asked of it?) For our current purposes, the most crucial thing Kant denies is that thought can be bound by external standards. The standard that thought has to correspond to reality must, according to Kant, be a standard internal to thought itself.

Kant’s crucial commitment can be formulated as follows: thought is normative, and the standards to which it answers must be its own; judgement is therefore the autonomous, free act of the mind (or of self-consciousness). This is what the empiricist tradition has failed to capture.

And this is why intuitions without concepts are blind. It is important to realise that for Kant, and not for the empiricists, concepts are rules. They are inherently normative. Using a concept is applying a rule, not following a causal law, and thus it is necessarily an act of the mind rather than something that merely happens to the mind.

At this point, it is useful to remind ourselves what Kant means by ‘intuition’ (‘Anschauung’). An intuition is the being in immediate contact with an object. Kant suggests that possibly God has ‘spontaneous’ intuition, intuition that is active and that creates its own objects. But we can’t even know whether such a thing is possible. Our intuition is sensible: we are in touch with objects when they appear to us in sensible experience. The problem with rationalism, then, is that it somehow does not do justice to the way we are in contact with objects. And if it fails to do justice to that, the result is that on the rationalist conception of thought, thought cannot even be about objects — and so cannot be either true or false. Again, we have a failure to so much as make judgement comprehensible.

What does the mistake of rationalism look like? We can start with the following question. Suppose I have a brilliantly worked out rational theory; it is coherent, simple, logically pleasing, and so on. Nice. But why would I think that it is true of the world? There’s nothing especially troubling about a rationalist theory of purely formal logic, which only sets out the forms of valid argument; and in fact, Kant accepts such a theory. But why would logical standards imply truths about the world? Even more fundamentally, how could such a theory even be about the world? How is the connection made? How do we leap out of the formal and into the objective? The connection must be made through intuition, through contact with objects; it is when we have intuition that we can say, ‘that thing, that is what this thought is about’.

Now the rationalist believes that we, in our thinking, are pale imitations of God. Our thoughts are like the thoughts of God, only not as good. They are fallible, whereas those of God are infallible; they are limited, whereas those of God are unlimited; and they are unclear, whereas those of God are perfectly clear. But this means that when we arrive at clarity — which is what the rationalist project will help us do — then at least of the limited things that we are clear about, we can have knowledge. Why? Because we are thinking the thoughts of God; and, importantly, because God’s thoughts create or are reality. When we think the rationalist mode of operation through, we see that it can claim to be about the world only by making our thoughts coincide with those of God, such that God’s active intuition can be the connection to the world that we need.

Kant believes that this mode of operation is deeply mistaken. Our thoughts are not like those of God. Our finitude is not simply a lack of perfection, as Descartes thought, but a unique form — God, for instance, does not use concepts and does not conceive of things as being in space and time. Our thoughts are radically different from those that God might have; which, in fact, should not be called thoughts at all. (Kant tells us that God does not think.) But we don’t need to delve this deeply at the moment. The rationalist project cannot get off the ground for the simple reason that if we are to even think God as really existing, we already need to be able to engage in objective thought. We can’t even reach God in our thought, much less make use of His type of intuition, unless we are already able to think and judge about objects that are not our own thoughts. And for this we cannot rely on anything except our own type of intuition — which happens to be sensible. Thus, thoughts without content are empty. Unless we tie thought to the world using sensible intuitions, our thoughts cannot be about anything except themselves, and must this be a purely formal play of thought thinking itself.

We can formulate this criticism in a way that closely parallels our criticism of the empiricist. In order for objective thought to be true or false, the empiricist had to bring in a standard external to thought; but such a standard cannot be applied by us (but only, one might want to say, by God) and it would anyway not be normative for thought itself. The rationalist goes wrong in a parallel way. While she does not bring in a standard external to thought, she brings in a standard external to finite thought, that is, to our thought. Instead of positing as a norm that thoughts correspond to the world (as an empiricist might try), she posits that our thoughts ought to correspond to God’s thoughts. But, again, this standard cannot be applied by us (but only, one might want to say, by God) and it would anyway not be normative for finite thought itself, since it comes, after all, from outside.

Thus, neither empiricism nor rationalism can do justice to the normativity of thought — the fact that thoughts can be good or bad, true or false — because neither is able to understand thought as an autonomous activity. It is only when we bring sensible intuition together with concepts-as-rules that the judgement of beings such as us can be understood.

Of course this bringing together of active thought and sensible intuition generates its own puzzles. How is it possible that thought is bound only by its own standards, and that it also has to be adequate to the world? The answer to that is transcendental idealism, but that is a story for another time.