This chapter explains how people can think of knowledge without imagining that it is built up laboriously from the summation of particular observations. The Germans have given a special word to the historicity of science: Theoriendynamik, the dynamics of theories. With the conception that knowledge has a historical dimension and that the theories knowledge consists of have a dynamism and succeed one another, even though they are all theories about the same world, one can abandon the older idea that different theories are different 'appearances' or different portraits of reality. The substantive part of the sociology of knowledge deals with the actual content of knowledge and seeks to explain it as a function of social experience or social self-interest. The great founders of the sociology of knowledge, from Marx on, have all been Positivists of sorts and have therefore argued that people's knowledge of nature is exempt from sociological explanation because it can be explained satisfactorily in observational terms.