Faculty psychology views the mind as a collection of separate modules or faculties assigned to various mental tasks. The view is explicit in the psychological writings of the medieval scholastic Theologians, such as Thomas Aquinas.
It is also present, though more implicitly, in Franz Joseph Gall's formulation of phrenology, the now-disreputable practice of measuring personality and sensory traits by estimating brain mass of organs on one's head to determine ways to improve faults. However, faculty psychology has been revived in Jerry Fodor's concept of modularity of mind, the supposition that different modules autonomously manage sensory input and other mental functions.
Historical change
It is debatable to what extent the continuous mention of faculties throughout the history of psychology should be taken to indicate a continuity of the term's meaning. In medieval writings, psychological faculties were often intimately related to metaphysically-loaded conceptions of forces, particularly to Aristotle's notion of an efficient cause. This is the view of faculties which is explicit in the works of Thomas Aquinas:
...knowledge of things in our intellect is not caused by any participation or influence of forms that are intelligible in act and that subsist by themselves, as was taught by the Platonists and certain other philosophers who followed them in this doctrine. No, the intellect acquires such knowledge from sensible objects, through the intermediacy of the senses. However, since the forms of objects in the sense faculties are particular, as we just said, they are intelligible not in act, but only in potency. For the intellect understands nothing but universals. But what is in potency is not reduced to act except by some agent. Hence there must be some agent that causes the species existing in the sense faculties to be intelligible in act. The possible intellect cannot perform this service, for it is in potency with respect to intelligible objects rather than active in rendering them intelligible. Therefore we must assume some other intellect, which will cause species that are intelligible in potency to become intelligible in act, just as light causes colors that are potentially visible to be actually visible. This faculty we call the agent intellect, which we would not have to postulate if the forms of things were intelligible in act, as the Platonists held.
By the 19th century, the founders of Experimental Psychology had a very different view of faculties. In this period, Introspection was well-regarded by many as one tool among others for the investigation of mental life. In his Principles of Physiological Psychology, Wilhelm Wundt insisted that faculties were nothing but descriptive class-concepts, meant to denote classes of mental events which could be discerned in introspection, but which never actually appeared in isolation. He took caution in insisting that older, metaphysical conceptions of faculties must be guarded against, and that the scientist's tasks of classification and explanation must be kept distinct:
It is probable that the mental faculties stood originally not merely for different parts of the field of internal experience, but for as many different beings; though the relation of these to the total being, the mind or spirit, was not conceived of in any very definite way. But the hypostatisation of these concepts lies so far back in the remote past, and the mythological interpretation of nature is so alien to our modes of thought, that there is no need here to warn the reader against a too great credulity in the matter of metaphysical substances. Nevertheless, there is one legacy which has come down to modem science from the mythopoeic age. All the concepts that we mentioned just now have retained a trace of the mythological concept of force; they are not regarded simply as -what they really are- class-designations of certain departments of the inner experience, but are oftentimes taken to be forces, by whose means the various phenomena are produced. Understanding is looked upon as the force that enables us to perceive truth; memory as the force which stores up ideas for future use; and so on. On the other hand, the effects of these different' forces' manifest themselves so irregularly that they hardly seem to be forces in the proper sense of the word; and so the phrase 'mental faculties' came in to remove all objections. A faculty, as its derivation indicates, is not a force that must operate, necessarily and immutably, but only a force that may operate. The influence of the mythological concept of force is here as plain as it could well be; for the prototype of the operation of force as faculty is, obviously, to be found in human action. The original significance of faculty is that of a being which acts. Here, therefore, in the first formation of psychological concepts, we have the germ of that confusion of classification with explanation which is one of the besetting sins of empirical psychology.
It was in this and the ensuing period that faculty psychology came to be sharply distinguished from the act psychology promoted by Franz Brentano -- whereas the two are barely distinguished in Aquinas, for example.