This led to the most important part of Kant's ethics, the formulation of the categorical imperative, which is the criterion for whether a maxim is good or bad.
Simply put, this criterion amounts to a thought experiment: to attempt to universalize the maxim (by imagining a world where all people necessarily acted in this way in the relevant circumstances) and then see if the maxim and its associated action would still be conceivable in such a world.
For instance, holding the maxim kill anyone who annoys you and applying it universally would result in self termination.
Kant's ethics focus, then, only on the maxim that underlies actions, and judges these to be good or bad solely on how they conform to reason.
For example, John Rawls[4][5] drew heavily on his inspiration in setting out the basis for a liberal view of political institutions.