All of Brentano's students recall the burning sense of a mission to render philosophy a science that permeated the core of their mentor's being and teaching, but what kind of science does Brentano defend for philosophy in his Psychology from an Empirical Standpoint (1874) and new science of descriptive psychology which he subsequently elaborated in his lectures in philosophy at the University of Vienna in the 1880s? Brentano would like us to believe that he is continuing the perennial science of philosophy as inaugurated by the ancient Greeks, and by Aristotle in particular, into the nineteenth century. This article, however, argues that once Brentano accepts the status of the newly emerging natural science of psychology and adopts in the 1870s a Humean philosophical approach to the mind and its contents, he cannot, despite his best efforts, establish the continuity of philosophy as philosophia perennis that exists before and after the modern conception of natural science. He thus defends for the science of philosophy instead, a psychology from an empirical standpoint that is fully modern in temperament, yet one that is neither fully empirical or rational in Hume or Kant's sense nor a capitulation to the method of the natural sciences in general and fledgling natural science of empirical psychology in particular.