Critique of Practical Reason
The "Groundwork" is broken into a preface, followed by three sections. Kant's argument works from common reason up to the supreme unconditional law, in order to identify its existence. He then works backwards from there to prove the relevance and weight of the moral law. The book is famously obscure, and it is partly because of this that Kant later, in 1788, decided to publish the "Critique of Practical Reason".