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 _à 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. 'I am myself inclined (p. 285) to admit unconscious mental modifications, in the only sense in which I can attach any very distinct meaning to them--namely, unconscious modifications of the nerves. It may well be believed that the apparently suppressed links in a chain of association, those which Sir W. Hamilton considers as latent, really are so: that they are not even momentarily felt, the chain of causation being continued only physically--by one organic state of the nerves succeeding another so rapidly, that the state of mental consciousness appropriate to each is not produced.'