Groundwork of the Metaphysic of Morals
Wow...I wouldn't normally try to give "the" summary for Kant's Groundwork. If you haven't read this -- read it! Kant asks what it would mean if morality exists. Unpacking this definition, he thinks that if morality exists, it has something to do with conformance to universal laws, and that if morality if binding for human beings, human beings have to be autonomous. For whether he thinks he can show that human beings are autonomous...read treatise 3!