‘By parity of Reason’: Universalizability, Impartiality and Reciprocity in Cumberland’s Theory of Natural Law

Abstract According to Richard Cumberland, men’s natural law obligation to promote the common good does not derive from self-interest, but there is really no conflict between private and public happiness because the former is contained in the latter. The aim of this paper is to disentangle the various arguments supposed to support this claim and to focus specifically on the ‘parity of reason’ argument which draws upon the principle of treating like cases alike. I will show that Cumberland tends to confuse the principle with a more substantive Golden Rule-like principle requiring that we treat others like ourselves and that he appeals to ideas of both impartiality and reciprocity in doing so. Moreover, I will argue that Cumberland’s failure to fully grasp the distinction between these three concepts derives from the way in which the overall structure of his argument integrates the perspective of human rational agents with the perspective of God.