The last couple of moral theories we covered were what philosophers call consequentialist theories. They are interested in the consequences of actions. Utilitarians care about whether an action causes more pleasure or more pain, and egoists care about whether an action satisfies one’s own interests. But worrying only about consequences misses out on a feature of actions that a lot of people seem to care about: whether the action was done with good intentions.
If you’re sympathetic to that idea, then you have to figure out what would count as “good” intentions. For the purpose of this discussion, let’s say that you have certain moral duties, and that if you do things because you have a duty to, then you have good intentions. Like if you have a moral duty to help those in need, and you see a child drowning in a shallow pool, then you have a duty to help. If you try to rescue the child and you fail (you don’t get there in time or something), that’s okay because you had good intentions. You were trying to do your moral duty, and the important thing is that you tried to help because you had a duty to help.
So how do you know what your duties are? There has to be a way to figure out what things you should be trying to do. The German philosopher Immanuel Kant suggested that duties came from rationality. He thought we should do only what is rational because rationality is special; only humans possess it. So the way to have a good life is to embrace your humanity. The moral theory that is bases morality on duties that come from rationality is called Kantianism, after Kant.
The obvious question for kantians is “how do I know what is rational and what isn’t”? Kant, having thought about this quite a bit, had a system for figuring out what’s rational and what isn’t, and thereby for figuring out what your moral duties are. It was really just a test you could use to determine if you have a duty to do something or not. He said “Act only on that maxim which you could at the same time will that it be a universal law of nature”. Which, in simpler terms, means “only do what you would want everyone else to do”. Kant called this test the categorical imperative. An imperative is just a command, and “categorical” just means that it applies at all times, not because of it’s results, but because it is right. So a categorical imperative is a command that you must follow at all times for its own sake. Kantians always try to do only what they would want everyone else to do.
[T]here is an imperative which commands a certain conduct immediately, without having as its condition any other purpose to be attained by it. This imperative is categorical. It concerns not the matter of the action, or its intended result, but its form and the principle of which it is itself a result; and what is essentially good in it consists in the mental disposition, let the consequence be what it may. This imperative may be called that of morality. …
There is therefore but one categorical imperative, namely, this: Act only on that maxim whereby thou canst at the same time will that it should become a universal law. …
You might think at first that the categorical imperative is kind of like the golden rule. That’s a good observation. It reveals an idea that is very important to kantians: acting because of a rule. If you decide what to do based on a rule, you’re using rationality. You’re not acting to satisfy desires or avoid danger—something that non-rational animals (according to Kant) can do. You’re acting because of a rule. Kantians think that the rule, or duty, is the important thing, not the consequences of the action. The thing that makes actions right or wrong for kantians is whether you do it with the right intentions.
So when Kant says “Act only on that maxim which you could at the same time will that it be a universal law of nature”, he is talking about rules. A maxim is just a rule, law, or policy. So if you “act on a maxim”, that just means you use a certain policy to decide what to do. For example, you might have a policy of brushing your teeth before you go to bed. You might think “it’s about time for me to go to bed. I guess I better brush my teeth now,” and then you do. Of course, you don’t brush your teeth because you have that policy. You brush your teeth to avoid tooth decay. So for a maxim to be a moral duty, it has to be the case that you do it because you have a policy about doing it. Maybe you have a policy about helping people in need. When you see a child drowning in a shallow pool, you try to rescue her because you feel like it’s your duty. You believe that trying to rescue people is the right thing to do, not because of what might happen if you don’t, but because it’s your duty. So when Kant says “Act only on that maxim which…”, he means “decide what to do based on policies or rules, not based on consequences”.
Then the last part of Kant’s test is that bit about universal laws of nature, which sounds very complicated, but its not really. If a maxim (policy) were a universal law of nature, then everyone would follow it even if they didn’t want to. Think of gravity. Gravity is a law of nature. You have to obey the law (maxim) of gravity even if you don’t want to. (You might have ways to overcome gravity, but you’re still affected by it.) “Act only on that maxim which you could at the same time will that it be a universal law of nature” means “only do things because you are following a policy that you would want everyone to follow (even if they didn’t want to)”.
To get back to the issue of rationality, Kant says it would be irrational to do something that fails his test. Suppose your friend asks you if you like a particular movie, suppose you happen to know that it’s your friend’s favorite movie, and suppose also that the movie in question is in fact completely awful. You might lie to your friend and say the movie is okay to spare his feelings. And that would seem like the morally appropriate thing to do. It wouldn’t accomplish anything to insult your friend’s poor taste in film, and it’s not as though his enjoyment of it is hurting anyone else. Is it your moral duty to lie, or is it your duty to tell the truth? Put it to Kant’s test. Are you lying because you have a policy about lying in order to spare someone’s feelings? And would you want everyone to do the same (even if they didn’t want to)?
Think about what would happen if everyone did that. Before too long, no one would ask their friends for opinions, because they’d know the answer would probably be a lie. Any lying would be like that. Lying only works (that is, you can only succeed in deceiving someone) if people think you’re not lying. But if everyone lied all the time as if it were a law of nature, then your friend would know that you’re lying. In fact, no one would believe anything anyone else said, and the very idea of lying to deceive someone would become nonsensical. It is in that sense, Kant thinks, that lying is irrational, and therefore you have a moral duty not to do it.
Killing works the same way. A kantian would say that you have a duty never to kill because killing is inherently irrational. If everyone killed all the time, as if it were a law of nature, then everyone would eventually be killed. No one could want that, so no one should have a policy that calls for killing. So now you can see how morality can come from rationality. Kantians say that you have a moral duty to do only what is rational, and you have duties to do those things because they are rational. As long as you intend to do your duty—that is, you have a good will—you are acting permissibly. And if you intend to ignore a duty, you are acting impermissibly.
A good will is good not because of what it performs or effects, not by its aptness for the attainment of some proposed end, but simply by virtue of the volition; that is, it is good in itself, and considered by itself is to be esteemed much higher than all that can be brought about by it in favour of any inclination, nay even of the sum total of all inclinations.
The trouble with kantianism is the opposite of the trouble with utilitarianism. Whereas utilitarians care only about consequences, kantians care only about intentions. Kantians don’t think it matters what happens as a result of your actions, as long as you do them out of duty. If, for example, in the course of trying to save the drowning child, you accidentally cause the child to drown, that’s okay because you were trying to do your duty. Your intentions were good. Nothing else matters. This causes real problems when a duty demands that you do the opposite of what you would think is morally good. Imagine that an axe murderer comes to your door and asks if your friend Jane is inside because he is hoping to chop her to bits. And suppose that in fact, Jane is inside, but the murderer has no way of knowing that. You would think that you should lie to him to protect your friend’s life, but you have a duty never to lie, because a universal maxim that allows lying would result in axe murderers, out of mistrust of whomever answers the door, simply barging in and finding the friend anyway. That’s an extreme case, and not very realistic, but the point is that it seems like there are at least some times when lying should be morally permissible, but kantians have to say that it’s never allowed. Likewise, there are probably lots of policies you can think of that you would want people to follow most of the time, but just not all the time. You might think that the consequences matter at least a little.
 Kant, Immanuel, Fundamental Principles of the Metaphysics of Morals, Project Gutenberg edition, 2004, section 2. http://www.gutenberg.org/cache/epub/5682/pg5682.html