Planning is concerned with rational mechanisms, and in it optimal form it works like a machine. In fact planned regulation implies the identification of key positions in the structure of interrelated principia media. The theory of social planning as formulated by Mannheim, and its foundations in his sociology of knowledge, can be seen as a solution to the problem of relativism with respect to social intervention. Time after time predictions and forecasts, no matter how sophisticated, turned out to be daydreams or fictions. The problem of cognitive relativism and planning rationality is no longer a problem. Planning, understood as a kind of rational thought and rational action, both interconnected, depends on a cognitive structure that can transcend the present situation. There must be a human design of the future of society, a point of reference that cannot be too relativistic.