Kant’s Ethics
Kant says that:
Nothing is absolutely good except a good will. The will is practical reason, i.e. the ability to act according to principles. So the best will (the purest will or willest will) is the most principled or rule-governed will. A good will is a rational will. What does a rational (i.e. good) will will? Rationality or principledness itself, obedience to exceptionless law.
The moral law: "I am never to act otherwise than so that I could also will that my maxim should become a universal law."
A maxim: a principle or motive of action.
The will is subject to (i.e. can be moved by) subjective impulses and objective principles. Nothing done on impulse, even a good impulse, has moral worth. Only actions performed on principle, out of a sense of duty, have genuine moral worth or value.
Rational action, action in which the will obeys the command of reason, can be of two kinds:
a) Action according to a hypothetical imperative, or b) Action according to a categorical imperative.
Hypothetical imperatives are based on subjective desires or preferences. E.g. if you want to lose weight, then you should go on a diet. A categorical form of this command or imperative would be: Go on a diet!
In fact, though, there is only one categorical imperative, only one thing that every rational being must do. This one thing can be expressed in different ways, e.g.:
"Act only on that maxim whereby you can at the same time will that it should become a universal law."
"Act as if the maxim of your action were to become by your will a universal law of nature."
"So act as to treat humanity [i.e. rational nature], whether in your own person or in that of another, in every case as an end. . ., never as means only. . .."
In other words, if you want to know whether an action is morally OK, follow these rules:
1) Think what principle the act in question is based on
2) Ask yourself whether everyone could (in theory) act in that way: if not, the act is irrational and bad
3) Ask yourself whether a truly rational being could want everyone to act that way: if not, the act is irrational and bad
Kant gives 4 examples of the categorical imperative in action, involving duties to self, duties to others, perfect duties (i.e. duties that apply all the time) and imperfect duties (i.e. duties that we only have to obey some of the time).
1) Perfect duty to self: Do not commit suicide.
Maxim: "From self-love I adopt it as a principle to shorten my life when its longer duration is likely to bring more evil than satisfaction."
Q. Could everyone do this?
A. No. The "special nature" of self-love is the promotion of self-preservation and self-improvement. A system of nature in which self-love led to self-destruction would be contradictory.
2) Perfect duty to others: Do not make lying promises.
Maxim: When it suits me, make a promise that I have no intention of keeping.
Q. Could everyone do this?
A. No. If everyone did this, promising would be meaningless. "I promise" would just mean "maybe." So there would be no point in promising: you would not be believed. So promising as such would disappear. If there were no promises, there would be no lying promises.
3) Imperfect duty to self: Improve your natural talents.
Maxim: Live a life of idleness, amusement and sex.
Q. Could everyone do this?
A. Perhaps.
Q. Could this be rationally willed?
A. No. Mental and physical powers are useful to you. It is irrational not to develop them and let them wither away.
4) Imperfect duty to others: Be charitable.
Maxim: Never be kind or helpful to anyone else.
Q. Could everyone do this?
A. Yes.
Q. Could this be rationally willed?
A. No. Everyone needs love, sympathy, kindness, etc. sometimes. Willing that these were never given would be willing your own despair.
So all 4 duties hold: breaking them is wrong.