He called these sentences categorical imperatives . Kant was silent on the details of how we come to know these moral absolutes. Since a noumenon is strictly unknowable, however, we cannot know how the noumena give us our knowledge of moral absolutes. Once we have picked up on those absolutes, though, we can then discover by reasoning all the ways in which they should govern our deedsin real life. Compassion, for instance, has no legitimacy as a source of moral judgement.
Hypothetical imperatives are unproblematic.
They are straightforward sentences that express mundane statements of fact. Categorical ones, on the other hand, are highly problematic. Certainly their philosophical significance is hard to grapple with. But, I believe, they are hard to understand at all. People who count themselves as Kantians often give answers along the following lines.