_Comparative Value of this kind of Knowledge._--The knowledge thus given in conscious experience is more correct and reliable than any other. It has this peculiarity that it cannot be disputed. I may be mistaken in regard to the properties of a piece of matter which I hold in my hand, and which seems to me to be square or round, of such or such a color, and of such or such figure, size, and density; but I cannot be mistaken as to the fact, that it _seems_ to me to be of such color, figure, etc. The former are results of perception and judgment; the latter is an immediate datum of consciousness, and cannot be called in question. To doubt our own consciousness is to call in question our very doubt, since the only evidence of our doubting is the consciousness that we doubt. As to the phenomena of the external world--the things that are passing without--I may be mistaken; as to what is passing in my own mind--the thoughts, feelings, volitions of my own conscious self--there is no room for doubt or mistake. _Not limited to Consciousness._--I do not mean, by what has been said, to imply that in our own observation of mental phenomena we are limited to the experience of our own minds, but only that this is the principal source of our information. The mental operations of others, so far as we have access to their minds, are also legitimate data. These we may observe for ourselves in the daily intercourse of life, may notice how, under given circumstances, men will think, feel, and act, and the knowledge thus acquired will constitute a valuable addition to our self-knowledge. We may receive also, in this science, as in any other, the testimony of others as to their own mental states and operations. In so far as psychology relies upon these sources, it stands on a footing with other sciences. § II.--IMPORTANCE OF MENTAL SCIENCE. _Comparative Neglect._--That the science of the mind has not hitherto held that high place in the public regard and estimation, at least in our own country, to which it is justly entitled, as compared with other branches of knowledge, can hardly be denied. The cause of this comparative neglect is to be found partly in the nature of the science itself, partly in the exclusively practical tendencies of the age.