It looked as if the great scientific discoveries of the 17th century, and in particular Isaac Newton’s scientific discoveries, had shown that there's no such thing as free will. But actually that pessimistic conclusion was not Kant's. Kant thought that despite all of this it was possible for us to acknowledge exercises of a free will, and to make sense of the idea that we were free agents. To understand Kant’s reconciling project here we have to go back to the very basic structure of his philosophical system, which can be understood in terms of a popular metaphor that's often used to characterise that system.

Kant’s view was that each of us carries around with us something like a metaphorical pair of spectacles through which we view things. Because we view things through these spectacles, we're only ever aware of appearances and not really how things are in themselves beyond the spectacles. We can't do with these metaphorical spectacles what we can do with a literal pair of spectacles, which is to take them off and look at things directly. There's no such process in the case of the metaphorical spectacles. So Kant is working with a very profound and very important distinction between how things appear through the spectacles and how they really are in themselves. It's a very radical view. The mere idea that there's a distinction between appearance and reality is perhaps not so radical in itself, but Kant’s version of that idea is very radical because part of these native spectacles is space and time themselves.

Kant believed that the very fact that things appeared to us spatio-temporally is a fact about how things appear to us. Things in themselves are neither spatial nor temporal. We have absolutely no idea what they are like, but they're not spatial and they're not temporal.

So now we have the resources to understand how Kant can carry through his reconciling project, because what Kant said was, ‘Look, Newton and the other great scientists were concerned with the world of space and time.’ That's what they were investigating. They were investigating the world of appearances. The fact that everything is inexorably determined in the world of space and time is only a fact about the world of space and time. Any physical occurrence, anything that happens in time and anything that has a spatial location is a feature of the world of appearances. And at that level, there is no room for freedom; there are no exercises of free will at that purely physical level. But in Kant’s view there is an underlying reality, that there are exercises of free will, and we ourselves are part of the underlying reality.

These are questions that only make sense on the assumption that we're free agents, so it was extremely important for Kant that he should leave room for the possibility that we are free agents. It's extremely important for him that he's been able to carry through his reconciling project. So Kant thinks that we can rest content with what each of us intuitively thinks anyway, which is that there is a difference between right and wrong. There is such a thing as acting morally as opposed to immorally.

Now, the question is: how do we tell what counts as the right thing to do? How do we determine how we should live on the assumption that we have free will and are in control of how we should live? It was Kant's very distinctive view that we could answer these fundamental questions about how to live in purely rational terms. A sheer exercise of reason is able to distinguish between what counts as the right thing to do and what counts as the wrong thing to do.

Suppose that I've made a promise to somebody and I'm tempted to break that promise for one reason or another. I've reached the point where I have to decide whether or not to fulfil the promise. There are various reasons why it would be significantly in my own self-interest not to keep the promise, and I'm wondering whether just this once I can get away with doing what I know is normally the wrong thing to do.

Kant said, ‘Okay, think about this. If it really is legitimate for me to break my promise on this occasion, then it would be legitimate for anybody to break their promise in similar circumstances.’ The whole point is that the basic, fundamental precepts of morality don't vary from one individual to another or from one situation to another. So I've got to ask myself: what would it be like if everybody decided to break their promise when it suited them for selfish reasons? Kant said that if you think about it you'll soon realise that the whole institution of promising would disintegrate in those circumstances. So there you are seeing a way based on a pure exercise of reason why it's illegitimate ever to break your promises.