FUNDAMENTAL LAW OF PURE PRACTICAL REASON: So act that the maxim of your will could always hold at the same time as a principle in a giving of universal law.
This quote is Kant's famous categorical imperative. In the Analytic, this, the one ultimate ethical principle, is derived. The proof distinguishes the form of a law, which is that it is universally applicable, from its matter, what in particular it tells us to do. It then argues that if the matter is anything beyond an expression of the form, the law is no law, for obeying any such material principle would be heteronomous. The categorical imperative, or law of pure practical reason, is the only law we can follow and still be acting appropriately freely. This principle says that we can morally assess an action by imagining everyone in the world acting on its motive. If this thought is coherent, the action is right; if not, it is wrong. If a person commits suicide when he is tired of life, this is wrong, for if everyone disposed of his life so casually, the society the suicide depended on while the man lived would collapse. Similarly, if everyone refused to give charity, society would collapse. If everyone told lies, no one would be believed, so the very act of lying would be impossible; therefore, lying is not universalizeable, and so it is wrong.