Mr Mill then proceeds to distinguish these various meanings, and to determine in which of them the phrase is understood by Sir W. Hamilton. One meaning is, that we only know anything by knowing it as distinguished from something else--that all consciousness is of difference. It is not, however, in this sense that the expression is ordinarily or intentionally used by Sir W. Hamilton, though he fully recognizes the truth which, when thus used, it serves to express. In general, when he says that all our knowledge is relative, the relation he has in view is not between the thing known and other objects compared with it, but between the thing known and the mind knowing--(p. 6). The doctrine in this last meaning is held by different philosophers in two different forms. Some (e.g. Berkeley, Hume, Ferrier, &c.), usually called Idealists, maintain not merely that all we can possibly know of anything is the manner in which it affects the human faculties, but that there is nothing else to be known; that affections of human or of other minds are all that we can know to exist--that the difference between the ego and the non-ego is only a formal distinction between two aspects of the same reality. Other philosophers (Brown, Mr Herbert Spencer, Auguste Comte, with many others) believe that the ego and the non-ego denote two realities, each self-existent, and neither dependent on the other; that the Noumenon, or 'thing _per se_,' is in itself a different thing from the Phenomenon, and equally or more real, but that, though we know its existence, we have no means of knowing what it is. All that we can know is, relatively to ourselves, the modes in which it affects us, or the phenomena which it produces--(pp. 9--11). The doctrine of Relativity, as held by Kant and his many followers, is next distinguished from the same doctrine as held by Hartley, James Mill, Professor Bain, &c., compatible with either acceptance or rejection of the Berkeleian theory. Kant maintains that the attributes which we ascribe to outward things, or which are inseparable from them in thought, contain additional elements over and above sensations _plus_ an unknowable cause--additional elements added by the mind itself, and therefore still only relative, but constituting the original furniture of the mind itself--inherent laws, partly of our sensitive, partly of our intellectual faculty. It is on this latter point that Hartley and those going along with him diverge. Admitting the same additional elements, these philosophers do not ascribe to the mind any innate forms to account for them, but hold that place, extension, substance, cause, and the rest, &c., are conceptions put together out of ideas of sensation, by the known laws of Association--(pp. 12--14). Partial Relativity is the opinion professed by most philosophers (and by most persons who do not philosophize). They hold that we know things partly as they are in themselves, partly as they are merely in relation to us.