So much for the essentials of the cognitive relation, where the knowledge is conceptual in type, or forms knowledge 'about' an object. It consists in intermediary experiences (possible, if not actual) of continuously developing progress, and, finally, of fulfilment, when the sensible percept, which is the object, is reached. The percept here not only _verifies_ the concept, proves its function of knowing that percept to be true, but the percept's existence as the terminus of the chain of intermediaries _creates_ the function. Whatever terminates that chain was, because it now proves itself to be, what the concept 'had in mind.' The towering importance for human life of this kind of knowing lies in the fact that an experience that knows another can figure as its _representative_, not in any quasi-miraculous 'epistemological' sense, but in the definite practical sense of being its _substitute_ in various operations, sometimes physical and sometimes mental, which lead us to its associates and results. By experimenting on our ideas of reality, we may save ourselves the trouble of experimenting on the real experiences which they severally mean. The ideas form related systems, corresponding point for point to the systems which the realities form; and by letting an ideal term call up its associates systematically, we may be led to a terminus which the corresponding real term would have led to in case we had operated on the real world. And this brings us to the general question of substitution. IV. SUBSTITUTION In Taine's brilliant book on 'Intelligence,' substitution was for the first time named as a cardinal logical function, though of course the facts had always been familiar enough. What, exactly, in a system of experiences, does the 'substitution' of one of them for another mean?