According to Kant’s system, there are certain ways of acting that are always wrong, categorically wrong for any person, at any time, in any society. For example, it is always wrong to make a false promise, a promise which you do not intend to keep. When you make a false promise, you are acting on a principle that could not be adopted by everyone. It is absolutely impossible for everyone to adopt and act on a principle of making false promises; if everyone did, no one would trust anyone else, or believe that they would honour their promises. False promising would be impossible, because no one would accept your promise. When you make a false promise, you are relying on other people honestly keeping their promises; you are treating yourself as an exception.

If you lie to someone, or make a promise that you do not intend to keep, you treat others as means, not as ends. You may be lying to that person to benefit yourself, in which case you are certainly using him as a means. But for Kant, lying is wrong whatever reason you have for the lie. Kant is deeply opposed to utilitarian theories, according to which lying to someone to make him happier is entirely justified. Even if you are trying to benefit the person to whom you are lying by shielding him from the harsh truth, you are treating that person in a way to which he could not give consent. Lying to someone is trying to deceive them, trying to give them false beliefs about what you are really doing. On Kant’s view, that cannot be right.

Trusting relationships are important to Kant’s theory in several ways. Trust between people is indispensible as a means of acquiring other things of value. If we never trusted anyone, we could never learn anything useful from anyone else; after all, they might not be telling us the truth. Nor could we cooperate with other people in joint ventures; after all, they might fail to honour their side of the deal. But in Kant’s theory, trust has a crucial role, too, in expressing our respect for ourselves and others. We treat people with respect when we refuse to lie to them, and when we refuse to make promises that we do not intend to keep. We treat them with respect when we place our trust in them, and expect them to deal honestly with us. Trust is fundamental to the relationships of respect between the members of the kingdom of ends, the ideal moral community.

Kant’s theory tells us to act on principles that could be adopted in the kingdom of ends, an ideal community, in which no one lies or breaks their promises, and everyone trusts everyone else. But in our society, which is so far from this ideal, isn’t Kant’s theory hopelessly impractical? At times, Kant is indeed too idealistic: he says that we should not lie even to a murderer who is looking for our friend, his intended victim. In the real world, we have to appreciate that not everyone is trustworthy, and that some people will take advantage of our honesty. We have to learn to recognize those people with whom we will never be able to achieve relationships of respect and of trust. But when such relationships are possible, we must do our best to create and sustain them. We simply cannot afford not to. As Kant shows us, when trust breaks down, not only do we miss out on the benefits of cooperation, but we also lose something less tangible, but no less important: respect for one another.