All consciousness must in the first instance present itself as a relation between two constituent parts, the person who is conscious, and the thing, whatever it may be, of which he is conscious. This contrast has been indicated, directly or indirectly, by various names--mind and matter; person and thing; subject and object; or, lastly, in the distinction, most convenient for philosophy, however uncouth in sound, between self and not self--the ego and the non-ego.