The first critique (Critique of pure reason) is focused on judgments about experience. Pure practical is concerned with the a priori frounds for action, and, especially, moral action. Kant is trying to solve the ultimate moral question : What sould i do ?
Kant’s second critique
For Kant, in the second critique and other writings on ethics, such as Groundwork of the Metaphysics of Morals, this involves awareness of the moral law, and a motive for acting in accordance with that law. He claims that this motive, if we are concerned with pure reason, must be different in kind from any natural, empirically conditioned interest.
Kant makes it plain that he is not attempting to dismiss our ordinary moral judgments or to produce a new morality : “no new principle is set forth in it, but only a new formula“. Kant’s most famous contribution to moral philosophy does in fact appear to be a formula. This is the categorical imperative, the best known version of which is “Act only on that maxim which you can at the same time will to become a universal law“. (see Kant quotes)
This appears to provide us with the necessary, but not sufficient condition for a moral principle, namely, that one should only adopt and act on principles which everyone could adopt, but it does not tell us which of such principles we ought to adopt. So the categorical imperative is best seen not as a source of moral principles, but as a test of those principles we already have.
In calling his imperative ‘categorical‘, Kant contrasts it with hypothetical, which are imperatives we can choose to take note of, if we have a particular aim. A hypothetical imperative might be offered as prudential advice.
Kant and morals actions
If we motive action is not any particular wis, what is it ? For Kant, to act morally is to act for the sake of duty. Feelings and inclinations cannot be the motive for moral action, for however desirable and admirable they may seem to be, they are not, he claims, subject to the will.
These principles led Kant to maintain that it was never right to tell a lie : the obligation to be thruthful cannot be limited by any expediency.
Kant does allow one type if feeling as morally important, the feeling that awareness of the moral law, which he calls ‘respect’: a respect which is a sort of awe, expressed by Kant at the conclusion of the Critique of practical reason : “Two things fill the mind with ever new and increasing admiration and awe, the oftener and more steadily we reflect on them : the starry heavens above and the moral law within”
This work is definitely huge in Kant’s Philosophy.