built up from infancy to maturity; the process is always going on before our eyes, yet the stages of it—­especially the earliest stages, the most pregnant with instruction—­are never studied and put on record by observers trained in inductive logic, knowing beforehand what they ought to look for as the sine qua non for proving or disproving any proposed theory.  Such cases as that cited by Platner—­cases of one marked congenital defect of sense, enabling us to apply the Method of Difference—­are always within reach; but few Platners are found to scrutinize and record them.  Historians of science describe to us the laborious and multiplied observations, and the elaborate precaution for ensuring accuracy of observation, which recent chemical and physical inquirers have found indispensable for the establishment of their results.  We cannot, therefore, be surprised that mental philosophers, dealing with facts even more obscure, and careless about enlarging, varying, authenticating their records of particular facts, should have had little success in establishing any results at all.

But if even those, who adopt the psychological theory, have been remiss in the observation of particular mental facts,—­those who deny the theory have been far more than remiss; they have been blind to obvious facts contradicting the principles which they lay down.  Mr Mill, in chap, xiv., deals with this denial, common to Mr Mansel with Sir W. Hamilton.  That philosophers so eminent as both of them should declare confidently—­’what I cannot but think must be a priori, or original to thought; it cannot be engendered by experience upon custom’ (p. 264)—­appears to us as extraordinary as it does to Mr Mill.  Though no one ever surpassed Sir W. Hamilton in large acquaintance with the actual diversities of human belief, and human incapacities of believing—­yet he never seems to have thought of bringing this acquaintance into account, when he assured the students in his lecture-room, that custom, experience, indissoluble association, were altogether insufficient to engender a felt necessity of believing.  Such forgetfulness of well-known mental facts cannot be reproached to the advocates of the psychological theory.

In chap. xv.  Mr Mill examines Sir W. Hamilton’s doctrine on unconscious mental modifications.  He points out the confused manner in which Sir W. Hamilton has conceived mental latency, as well as the inconclusive character of the reasoning whereby he refutes the following doctrine of Dugald Stewart—­That in the most rapid trains of association, each separate item must have been successively present to consciousness, though for a time too short to leave any memory.  Sir W. Hamilton thinks that the separate items may pass, and often do pass, unconsciously; which opinion Mr Mill also, though not approving his reasons, is inclined to adopt.