In beginning these studies Dewey observes that certain idealistic writers (not named) have been employing in support of their idealism certain facts which have an obvious physical nature and explanation. Such illusions as that of the bent stick in the water, the converging railway tracks, and the double image that occurs when the eye-ball is pressed, have, as the realists have well proved, a physical explanation which is entirely adequate. Why is it that the idealists remain unimpressed by this demonstration? There is a certain element in the realistic explanation which undoubtedly explains the reluctance of the idealists to be convinced. "Many realists, in offering the type of explanation adduced above, have treated the cases of seen light, doubled imagery, as perception in a way that ascribes to perception an inherent cognitive status. They have treated the perceptions as _cases of knowledge_, instead of as simply natural events...."[251] Dewey draws a distinction, at this point, between naive and presentative realism, employing, by way of illustration, the 'star' illusion, which turns upon the peculiar fact that a star may be seen upon the earth long after it has ceased to exist. The naive realist remains in the sphere of natural explanation. He accounts for the star illusion in physical terms. The astronomical star and the perceived star are two physical events within a continuous physical order or process. But the presentative realist maintains that, since the two stars are numerically separate, the astronomical star must be the 'real' star, while the perceived star is merely mental; the real star exists in independence of a knowing subject, while the perceived star is related to a mind. The naive realist has no need of the hypothesis of a knower, since he can furnish an adequate physical account of the numerical duplicity of the star. Dewey favors the naive standpoint, and affirms that presentative realism is tainted by an epistemological subjectivism. "Once depart," he says, "from this thorough naivete, and substitute for it the psychological theory that perception is a cognitive presentation of an object to a mind, and the first step is taken on the road which ends in an idealistic system."[252] The presentative realist, Dewey continues, finds himself possessed of two kinds of knowledge, when he comes to take account of inference; for inference is "in the field as an obvious and undisputed case of knowledge." There is the knowledge of perception by a knower, and the inferential knowledge which passes beyond perception. All reality, consequently, is related, directly or indirectly, to the knowing subject, and idealism is triumphant. But the real difficulty of the realist's position is that, if perception is a mode of knowing, it stands in unfavorable contrast with knowledge by inference. How can the inferred reality of the star be established, considering the subjectivity of all perception?