Pneumatology, or, as it was also called, Rational Psychology, has been already alluded to in the Introduction to the Logic as an abstract and generalizing metaphysic of the subject. Empirical (or inductive) psychology, on the other hand, deals with the ‘concrete’ mind: and, after the revival of the sciences, when observation and experience had been made the distinctive methods for the study of concrete reality, such psychology was worked on the same lines as other sciences. In this way it came about that the metaphysical theory was kept outside the inductive science, and so prevented from getting any concrete embodiment or detail: whilst at the same time the inductive science clung to the conventional common- sense metaphysics with its analysis into forces, various activities, etc., and rejected any attempt at a ‘speculative’ treatment.
The books of Aristotle on the Soul, along with his discussions on its special aspects and states, are for this reason still by far the most admirable, perhaps even the sole, work of philosophical value on this topic. The main aim of a philosophy of mind can only be to reintroduce unity of idea and principle into the theory of mind, and so reinterpret the lesson of those Aristotelian books.