Both documents show not only that Kapp's critique of private enterprise was too unbalanced for Clark's taste, but also that Clark's own views on the role of private enterprise had changed since the 1920s. Since the draft of the manuscript reviewed by Clark is not available, we do not know in how far Kapp modified his critique. In this context it is of interest that Kapp's letter to Clark on December 12,1945 referred to a working title that differs from the later book publication: “I am very grateful to you for having read parts of my manuscript on Social Costs and Returns which I submitted to you” We do not know whether Clark's influence was decisive on Kapp's decision not to publish roughly half of his original book manuscript, which contained a discussion of planning for social returns of untilities which diffuse themselves among all members of society. that are to a large extent inappropriable by individual producers and cannot always be appraised in monetary terms" (Kapp, unpublished ms.III). Kapp had originally developed the interconnected themes of social costs and returns in his dissertation (Kapp 1936).