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 progress of society like that of the individual depends, then, ultimately on choice. It is not “natural,” in the sense in which a physical law is natural, that is, in the sense of going forward automatically from stage to stage without backward turnings, deflections to the left, or fallings away on the right. It is natural only in this sense, that it is the expression of deep-seated forces of human nature which come to their own only by an infinitely slow and cumbersome process of mutual adjustment. Every constructive social doctrine rests on the conception of human progress. The heart of Liberalism is the understanding that progress is not a matter of mechanical contrivance, but of the liberation of living spiritual energy. Good mechanism is that which provides the channels wherein such energy can flow unimpeded, unobstructed by its own exuberance of output, vivifying the social structure, expanding and ennobling the life of mind.