The first formulation (Formula of Universal Law).

[diamantis]

"Always act according to that maxim whose universality as a law you can at the same time will", and is the "only condition under which a will can never come into conflict with itself…"

Immanuel Kant.

[Jazzman]

I recently read the "Grundlegung zur Metaphysik der Sitten", Kants first book about his ethics. In my opinion it's very confusing. He doesn't make clear what's the role of "acting by duty". The first chapter suggests that duty shall be the only motivation of our acting. But in the second chapter he describes the use of the categorical imperative just as a test procedure for decisions in situations you're in due to already made [irrational] decisions. Additionally Kant doesn't really explain the connection between the general formulation and the "law of nature" formulation resp. he stays very notional.