The Critique of Practical Reason

In "The Critique of Practical Reason", Immanuel Kant argues that principles are subjective (and therefore maxims) if one person considers them; they are objective (and therefore imperatives) if every rational being considers them. Imperatives are either hypothetical or categorical. A hypothetical imperative demands a course of action to achieve a specified result; for example, “If I want to stay dry in the rain, then I should take my umbrella with me.” A categorical imperative demands a course of action under all possible circumstances; for example, “Thou shalt not commit murder.”
According to Kant, hypothetical imperatives respond to desires, while categorical imperatives constitute rationality.

Kant is often described as an ethical rationalist, and the description is not wholly inappropriate. He never espoused, however, the radical rationalism of some of his contemporaries nor of more recent philosophers for whom reason is held to have direct insight into a world of values or the power to intuit the rightness of this or that moral principle.