This chapter outlines Ludwik Fleck's philosophy and sociology of scientific knowledge and employs that approach to provide a new perspective on Arthur Cecil Pigou's economic thinking relative to Alfred Marshall. The various characteristics and attributes of Pigou's life and contributions that are identified in Chaps.  2 and  3 are considered from the perspective of Fleck's notion of "thought collective" and the related, but different, notion of "thought style". These distinctions are then employed to develop an alternative and largely consistent way of understanding the concept of "Marshallian" economics and to identify mechanisms to account for the "Marshallian" thought style that evolved under Pigou's influence. In this way, the Fleckian framework provides a means to interpret adaptation and modification in the "Marshallian" thought style as part of an evolutionary process.