In matters moral, reason is weak, easily suborned by the passions, given to rationalization, and easily entangled in the threads of its own dialectic. Reason is not to be despised but not quite reliable. In matters moral, it is better to rely on conscience.
This advice rests on two presuppositions. One is that conscience is a source of moral knowledge, which itself presupposes that there is moral knowledge. The other is that one's conscience has been well formed. Both presuppositions need examination. But don't make the reliance on conscience contingent on the completion of their examination.