Benefiting oneself at another's expense harms a self with whom one is intimately linked in Reality.

At first Russell was an adherent of McTaggart. This afforded him a neat solution to Sidgwick’s problem. The happy day when the harmony becomes explicit can be promoted or retarded by human action. If I benefit myself at your expense not only am I doing down a self with whom I am, in Reality, intimately linked—I am putting off the day when the harmony that Really Is becomes apparent. And since this harmony will be supremely pleasurable I am harming myself into the bargain. Hence morality and self