Friedrich Nietzsche's criticism towards the substance-concept I plays an impor-tant role in his late thought, and can be properly understood by making reference to the 19th century debate on the scientific psychology. Friedrich Lange and Ernst Mach gave an important contribution to that debate. Both of them developed the ideas of Gustav Fechner, and thought about a psychology without a soul, i.e. an investigation that gives up with the old metaphysics of substance in dealing with the mind-body problem. In this paper I shall deal with Lange's and Mach's view of the I/soul, in order to shed some light on Nietzsche's rejection of the I in philosophy.