When you read my Remarks again, you will observe, that I place morality solely and entirely on the nature, relations, and fitness of things; for I cannot conceive how any other principle can have the least share in the foundation of virtue. But perhaps you meant our obligation to the practice of moral virtue, which is a distinct consideration; and that I do indeed place upon a threefold bottom, the fitnesses, of things, the moral sense, (not a blind instinct) and the will of God: but interest is no part of the ground of moral obligation in my judgment; for what has that to do with conscience?
Catherine Cockburn (letter of 20 November 1744), in Catherine Trotter Cockburn: Philosophical Writings, Sheridan, ed., Broadview (2006) p. 234. The Remarks referred to include a review of Warburton's Divine Legation of Moses. The threefold bottom here is the same as Warburton's threefold chain, but she is shifting the emphasis. In Warburton's version of divine command theory, moral obligation strictly and properly consists in the sanction of a superior; but he also holds that God has established in us a moral sense and a rational ability to perceive relations of fitness between things as supplementary. (This is Warburton's attempt to assimilate the two major approaches to moral philosophy of the time, the sentimentalist, which grounded obligation on the feelings of a moral sense and taste, and the rationalist, which grounded obligation on the recognition of eternal truths.) Cockburn has a structurally similar view, but in her view it is rational perception of fitnesses that grounds moral obligation, with the sanction of the superior as supplementary.