Every effort of genuine science makes for a knowledge of the truth. Our only real and valuable knowledge is a knowledge of nature itself, and consists of presentations which correspond to external things. We are incompetent, it is true, to penetrate into the innermost nature of this real world—the “thing in itself”—but impartial critical observation and comparison inform us that, in the normal action of the brain and the organs of sense, the impressions received by them from the outer world are the same in all rational men, and that in the normal function of the organs of thought certain presentations are formed which are everywhere the same. These presentations we call true, and we are convinced that their content corresponds to the knowable aspect of things. We know that these facts are not imaginary, but real.

All knowledge of the truth depends on two different, but intimately connected, groups of human physiological functions: firstly, on the sense-impressions of the object by means of sense-action, and, secondly, on the combination of these impressions by an association into presentations in the subject.