_They differ in one Respect._--In one respect, indeed, our means of information with regard to the two branches of science differ. While both matter and mind can be known only by the observation of the phenomena which they present, in mental science the field of such observation lies in great part within ourselves--the phenomena are those of our own present or former consciousness--the mind is at once both the observer and the object observed. This circumstance, which at first seems to present a difficulty, is in reality a great advantage which this science possesses over all others. _Apparent Difficulty._--The difficulty which it seems to present is this: How can the eye perceive itself? How can the mind, as employed, for example, in remembering, or judging, or willing, inspect its own operations, since the moment its attention is turned to itself it is no longer engaged in that operation which it seeks to inspect--is no longer remembering, or judging, or willing, but is employed only in self-observation? We admit that the mind, in the very instant of its exercising any given faculty, cannot make itself, as thus engaged, the object of attention. But the operations of the mind, as given in consciousness, at any moment, may be retained or replaced by memory the next moment, and as thus replaced and attested, may stand before us the proper objects of our investigation, so long as we please. This puts it in the power of the mind to observe and to know itself. _Real Advantage._--The advantage accruing from the circumstance that the phenomena to be observed are those of our own present or former consciousness, is this: that those phenomena are fully within our reach, and also are capable of being known with greater certainty. In physical science the facts may be scattered over the globe, and over centuries of time, not personally accessible to any one observer in their completeness, and yet that completeness of observation may be essential to correct science. In psychology, the observer has within himself the essential elements of the science which he explores; the data which he seeks, are the data of his own consciousness; the science which he constructs is the science of himself.