[54] _The Nation_ quotes the following passage from a former essay of mine: "The philosophy of the future will be a philosophy of facts, it will be _positivism_; and in so far as a unitary systematisation of facts is the aim and ideal of all science, it will be _monism_." _The Nation_ rejects this definition of monism and adds: "The search for a unitary conception of the world or for a unitary systematisation of science would be a good definition of _philosophy_; and with this good old word at hand we want no other." Very well. Call that which we call monism or a unitary systematisation of knowledge, "philosophy"; we will not quarrel about names—_dummodo conveniamus in re_. We agree perfectly with our critic; for we also maintain that monism (at least, what we consider monism) is philosophy; it is _the_ philosophy. What then is the criterion of truth for a single fact, be it a sensation, a perception, or an observation? It is this, that if the observation be repeated under the same circumstances it will, to the extent that the circumstances are the same, be again the same; the observer will always make the same observation. This maxim will do for a statement of facts. If according to this maxim we are in the position to ascertain that the same observation can be made again and again under certain conditions, we gain the assurance that we have to deal with a fact of some kind. But how shall we inquire into the correctness of the interpretation of the fact? * * * * * Every living creature and furthermore among human beings every individual man has an idiosyncracy of his own. How can we avoid the errors arising therefrom? We substitute other observers so that we can detect to what extent the individual way of observation influences the result of the experiment. Thus we shall find that some persons are color-blind with reference to red or to green, and we can in this way explain certain mistakes caused by such conditions. Supposing that all human beings were color-blind we should consider this state as normal; and the discovery of science that certain colors which appear alike to us, are after all, considering their wave-lengths and other qualities, more different than certain other tints which are easily discerned by the eye, would be an unexpected surprise. It would to some extent be analogous to the well-known fact that there are rays of light which are not perceptible to the eye, namely, the so-called chemical rays; their existence has been discovered by their chemical effects.