By reading Kant.
I never figured why that guy got any traction. It seemed to me philosophy classes devoted to studying Kant were little more than group mental masturbation, missing the entire point of the biologic contribution to why we are what we are and why we behave how we behave, and substituting instead fancy blather not based on evidence or practical reality of any kind.
He just made stuff up, in short. Not that he was alone in that approach.