Critique of Practical Reason

“Kant's second Critique shows that pure reason can be practical: that the moral law, which commands unconditionally, is a 'fact of reason' through which we know ourselves to be free. Duty, not happiness, is the ground of morality; respect for the law is its only motive; and the hope that virtue and happiness may at last be united leads reason to postulate immortality and God. It ends with the starry heavens above and the moral law within.”