In spite of some disagreement with Neville Keynes on various methodological issues, from the second to the fourth editions of Principles, Marshall referred the reader to Keynes's The Scope and Method of Political Economy "for a more full and detailed investigation of the subject". As time passed, however, Marshall became increasingly aware that an incorrect interpretation of the hypothetico-deductive method, which he considered to be in itself essential to the statical and "preliminary" stage of economic analysis, was becoming a hindrance to fair reception of his own research programme, aimed at the development of "economic biology".