_Difficulty of such an Investigation._--The importance of such a preliminary investigation is scarcely greater than its difficulty. It would be easy, indeed, to mention, almost at random, a considerable number of mental operations, with whose names we are familiar; and a little thought would enable us to enlarge the list almost indefinitely. But such a list, even though it might chance to be complete, would be neither an analysis nor a classification of these several powers. It would neither teach us their relations to each other and to the whole, nor enable us to understand the precise nature and office of each faculty. We could not be sure that we had not included under a common name operations essentially different, or assigned distinct places and offices to powers essentially the same. Much depends, moreover, on the order in which we take up the several faculties. It is evident at a glance that to form a clear, correct, and comprehensive arrangement of the powers of the mind, is no slight undertaking. A complete understanding of the whole science of the mind is requisite. It is one of the last things which the student is prepared to undertake, yet one of the first which he requires to know. Unfortunately for the science, perhaps no topic in the whole circle of intellectual investigation has been more generally neglected, by those who have undertaken to unfold the philosophy of the mind, than the one now under consideration. § I.--GENERAL ANALYSIS. _A mental Faculty, what._--In making out any scheme of classification, the question at once arises, how are we to know what are, and what are not distinct faculties? In order to this, we must first determine what constitutes a mental faculty. What, then, is a faculty of the mind? I understand by this term simply the mind's power of acting, of doing something, of putting forth some energy, and performing some operation. The mind has as many distinct faculties, as it has distinct powers of action, distinct functions, distinct modes and spheres of activity. As its capabilities of action and operation differ, so its faculties differ.