(3) The origin of the subjectivist fallacy, as has been brilliantly shown by Avenarius,[52] is to be found in the “intrasubjective intercourse” of a plurality of percipients capable of communicating their experience to each other. So long as I am dealing solely with myself as an experiencing being and my relation to my own environment, there is no possibility of a subjectivist interpretation. In my own direct experience I have to do neither with “mental states” nor with mere “objects of cognition,” but with _things_ which in various ways by their interference assist or hinder the accomplishment of my various purposes, and of which I have therefore to take note, so as to adapt my ways of reaching my ends to their ways of behaviour. Hence the “natural” view of the world, for a single experiencing being, would be that of “naïve realism,” to which the things forming my environment are real in precisely the same sense in which I am real myself. But as soon as I have to take account of the experiences of other percipients, there arises an inevitable fallacy which leads to philosophical consequences of the gravest kind. Starting with the assumption that the things I perceive are the real things, I feel a difficulty as to how the same things can be perceived by the other percipients around me. _E.g._, if the sun I see is the real sun, what about the sun seen by some one else? Instead of finding the true explanation, that all the percipients are in relation to a common environment which is independent of its presence to any one percipient’s experience, I very naturally fall into the mistake of thinking the things perceived by other men to be “ideas” or “percepts” of the real things perceived by me. These perceptual copies of the real things I, for obvious reasons, locate somewhere “in” the organisms of my fellow-percipients. Then I go on to interpret my own experience in terms of the theory I originally devised to meet the case of my fellow-men, and infer that what I myself perceive is a set of “percepts” or “ideas” produced “in” my organism by a reality “outside” all experience. And it is then an easy step to the final conclusion that, inasmuch as all known and knowable things are mere “ideas in some one’s head,” nothing else exists. Subjectivism is thus the last step in the development of the fallacy which begins with what Avenarius calls “introjection.” Just as we learned that the existence of our fellow-men is the cardinal fact of experience which affords the most immediate refutation of the subjectivist theory, so the original source of the subjectivist fallacy is failure to recognise their experience as being on the same level of reality as our own.