How do you know what action is right?

Through Kant's 2nd Proposition of Duty it is stated that an action done from duty has its moral worth, not in the purpose it attains- But rather from a formal principle also known as the maxim. The principle of doing one's duty whatever one's duty maybe. A maxim is a principle in which we act. It has two different parts: The Subjective Principle and The Objective Principle. The subjective principle is the principle upon which a moral agent does act. The objective principle is the principle upon which a moral agent would act. This means that the objective principle is the principle that the rest of the world, given that the rest of the world are rational agent, would follow. This could also be known as the universal rule. Kant does a good job describing why a right action is the right thing to do.