Kant believed that the meaning of concepts such as justics are a priori independent of and prior to human knowledge and they transcend human experience.
He believed that there are universal moral laws which are created by God. We have to use human reason to discover them.
This discovery starts with a series of maxims - subjective moral principles that, Kant believed, can be deduced by all rational human beings.
We can find out if a maxim is morally right by using the Catagorical Imperative. This has three tests which show whether a moral maxim is to be accepted as a universal law. They are:
The universal law principle - "act only on that maxim whereby which you can at the same time will that it becomes a universal law" (Kant).
The principle of humanity as an end not a means - "so act as to treat humanity, whether in your own person or in that of any other, in every case at the same time as an end, never as a means only" (Kant).
The principle of the universal kingdom of ends - "act according to the maxims of a member of a merely possible kingdom of ends legislating…