Kant

In Groundwork of the Metaphysics of Morals (1785), Immanuel Kant introduces us to the categorical imperative. The imperative is:

Act only according to that maxim by which you can at the same time will that it should become a universal law.

Kant views this as an objective, rational principle that must always be followed. A simple example is the concept of lying. Suppose there is a situation you find yourself in where you contemplate lying. You must ask yourself can your action (lying) be something everyone should do – universally? If you answer yes, then we reach an absurdity where everyone lies and there is no truth. The moral imperative is to never lie. Never.