The ethical theories of Augustine and Aquinas start with a basic assumption of a divinely revealed truth about human nature and the world. By the 18th century, the Age of Enlightenment, this starting point could no longer be taken for granted. Europe was still Christian without a doubt, but the age of secularization had set in. In medieval times God determined the law. For kant, “laws” are human constructions arrived at by reasoned public debate, and since reason is the only faculty we have that is “infallible” ( or so Kant hoped), the laws would always be right and just and therefore worth of our obedience.

Now that God couldn’t serve as our guide, at least for nonreligious people, Kant believed we needed to identify the fundamental principles of the human mind, on which we could base morality. He therefore proposes a philosophical system that distinguishes between different human faculties in order to identify the sole faculty, which can reliably serve a moral purpose. For him this is reason, and it must a separate category from all other faculties of the mind or heart, even positive ones like we saw in The Good Samaritan and Jumping Mouse such as love and empathy, or in Helen Keller such as passion, hope or good character. These and everything else about human nature was completely untrustworthy, anything to do with feelings or sensual desire was always ruled by self-interest and therefore unworthy of becoming a basis for morality. Reason alone is reliable and it is up to reason to act out of good will. Thus inclination, which falls outside reason is also unreliable. Just as the pursuit of happiness. Kant isn’t against Happiness per se, though, he’s simply believes one must be “worthy of happiness” by doing one’s duty to the public, e.g. making decision based on reason.

indeed deontological ethics differ from greek virtue ethics in that the latter is based on character as opposed to duty (deon) in greek. for kant it is not character but good will that counts, and the will must be autonomous from any feeling, even inclination.