As previously indicated, Kant saw a problem between the world of the physical sciences and morality.
In the world of the physical sciences, Newtonian physics ruled and all that occurs is the result of cause and effect. However, Kant understood that it this truth was applied in the realm of morality and ethics the inevitable problem of accountability resulted.
The basic tension existed in the context of the tension between personal freedom and determinism.
How can we be accountable morally for something that is outside of our freedom of choice?
There is the world of appearance and the world of things in themselves.
This put him in the position to say that there is the world of appearances and the physical sciences give us the whole truth about that. Kant validated the conclusions of the empirical sciences. According to Kant, Newton had gotten it right.
But bear in mind that we also are talking about the world of appearances. There is the topic of things in themselves and there is "room" there for other sorts of concepts altogether, such as free will, rational agency, good and bad.
There is room for these concepts not in the world of appearance but outside of the world of appearance.
Kant saw that these other matters, however, could not be topics of knowledge. Had we asked Kant, "Do you KNOW that there is free will" he would have responded that he did not know and that all he knew was that there was room for that possibility.
Ultimately, Kant believed that ethics comes out of reason. The conclusion of his moral philosophy is that he attempted to extract the essentials of morality from the pure concept of rationality.
For Kant, the essential quality of a moral agent is that he must be a rational agent. The essential requirements of morality are built into the concept of rationality itself. He attempted to show it seems that only a community of rational beings could consistently universally adopt a body of principles and action corresponding to our principles of morality.
This in turn led to Kant's Categorical Imperative in which he argued we should act according to that maxim which we would at the same time will that all individuals act and live by.
