 The analogy with mathematics is instructive, for we acquire our moral knowledge in the same way we acquire knowledge of mathematical axioms. We apprehend that 2+2 = 4 by apprehending 2+2 matches makes 4 matches and 2+2 balls makes 4 balls, and so on. We apprehend the algorithm in the particular cases after exposure to particular instances of its application, by a process of intuitive induction (FE 170). We apprehend it is prima facie right to keep promises by apprehending it is prima facie right to fulfill this or that particular promise. ‘What comes first in time is the apprehension of the self-evident prima facie rightness of an individual act of a particular type. From this we come by reflection to apprehend the self-evident general principle of prima facie duty’ (RG 33; also FE 170).