A posteriori - The opposite of a priori. Known by experience.
A priori - That which cannot be known without prior experience. For instance, it is a priori that bachelors are unmarried, because one does not need to go check anything in the world to know that this is so.
Autonomy - To be led by a rule, but not one imposed from without. Its opposite is heteronomy, being led from without. Kant understood freedom of the will as autonomy of the will. In the Critique of Practical Reason, he strives to show that the only way to act autonomously is to follow the moral law, and that whenever you follow the moral law, you thereby act autonomously.
Categorical Imperative - A rule for behavior that applies not hypothetically (depending on one's desires) but categorically (which is to say, universally and regardless of one's desires). It's opposite is a hypothetical imperative. Kant believes that only one rule fits this description: act always so that your maxim could hold universally. This rule is often referred to simply as "the categorical imperative."
Contingent - Something that does not necessarily have to be, something that is arbitrary. It is necessary that three and three add up to six. But it is only contingent that Kant was less than than six feet tall.
Empirical - That which can only be known by experience. The empirical world is the phenomenal world.
Heteronomy - Governed by an external rule. The opposite of autonomy.
Intelligible - The intelligible, or noumenal, world is the world as it is in itself. This is opposed by the sensible, or phenomenal world, the world as it appears to us. The fact that the world as it is in itself is not the world that appears to us is the main theme of Kant's Critique of Pure Reason. However, in the Critique of Practical Reason, Kant allows that our feeling for the moral law reveals to us both our noumenal autonomy and the truth of the noumenal postulates of pure practical reason.
Maxim - Rules which underlie one's actions. For Kant, there are two kinds of maxims, the categorical imperative, which is a law of reason, and all other maxims, which cannot be. The categorical imperative itself works by testing the maxim of an act, seeing whether it is universal in scope.
Noumenal - See intelligible.
Phenomenal - See intelligible.
Postulates of Pure Practical Reason - Those things that we must postulate in order to follow pure practical reason. Kant places God and immortality in this category. In the Critique of Pure Reason, both God and freedom were regarded as noumenal and thus neither provable nor disprovable. Here, we are given reason to believe in noumenal things, although Kant still insists that we cannot really understand them.
Pure Reason - Reason when it is not influenced by our contingent condition. Pure practical reasoning is an exercise of our decision-making capacities that does not involve our desires. Pure theoretical reasoning is an exercise of our representational capacities not based on our experiences.
Self-love - The faculty of seeking pleasure in the satisfaction of one's desires. According to Kant, there are two opposing ways of acting: according to any maxim which is not the categorical imperative, using self-love, or according to the categorical imperative, using pure practical reason. Kant opposes the idea that different sorts of desires have differing moral worth. If one acts altruistically because one likes to help, this, for Kant, means that one feels pleasure upon helping—that one's pleasure-seeking helps others is fortuitous, but at bottom it is pleasure-seeking self-love. Only acting according to the motive of duty can resist the label "pleasure-seeking."
Sensible - See intelligible.