But more than one current of thought runs through Butler's ethical treatise. The theological reference is sometimes so used as to make the obligation to morality, and even the nature of morality, depend on the will of God: though hardly according to Paley's crude method of seeking in the external revelation of the divine command a means of uniting the divergent interests of the individual and of society. In general, Butler's ruling idea is the idea of the system or unity of human nature, for which he was largely indebted to Shaftesbury's revival of the Platonic conception. Conscience is regarded by him as the expression of this unity. But its nature is never more deeply probed. Its deliverances are justified now by its supernatural mission, and now by the more prosaic fact that it leads to our individual interest[103]--at any rate, "if we take in the future"--while it could not be recommended as a guide if it did not.[104] On one side, therefore, Butler tends to a form of theological utilitarianism, such as was common in his own day, and was afterwards formulated by Paley.[105] On the other hand, his ethics more naturally allies itself with a different theory, in which the moral law is conceived as having its source in practical reason, and the naturalistic basis of ethics is definitely abandoned. [Sidenote: 4. The ethics of moral sentiment a mediating theory;] [Sidenote: explanation of its facts attempted by theory of evolution.] On the whole, it would appear that the psychological ethics worked out by Shaftesbury and his school occupies an insecure position between the view discussed in the two preceding chapters and that which ascribes to reason a function in the formation of objects of desire. Shaftesbury and his followers tried to strike out a middle course between the theory that ends of action may be determined by reason, and that which looks upon all desires as being desires for objects as pleasurable. They made the attempt to found a system of ethics on human nature, and they held that that nature could not be accounted for by the simple psychological analysis of the Epicurean school as then represented by Hobbes. On the other hand, they did not see their way to adopt the "rational" ethics only known to them in the abstract form it had received at the hands of Clarke and Wollaston. But their own theory of human nature requires a principle of harmony and co-ordination among the various impulses which they were unable to give a satisfactory account of. It may seem, however, that the idea of the development of man with which we are now familiar, may enable us to overcome the difficulties which formerly appeared insurmountable--showing the unity of human nature, and the tendency of its activity. The general course of evolution, to which all life has been subject, is thought to have brought about a harmony between individual and social feelings, as well as between individual