It is the historic and human interest that carries us back to these things. We say: What ingenuity! what a happy guess! how well that was reasoned in the light of what was actually known about the world in those days! But we never forget that what compels our admiration does so because it makes us realize that we stand in the presence of a great mind, and not because it is a foundation-stone in the great edifice which science has erected. But it is not so in philosophy. It is not possible to regard the philosophical reflections of Plato and of Aristotle as superseded in the same sense in which we may so regard their science. The reason for this lies in the difference between scientific thought and reflective thought. The two have been contrasted in Chapter II of this volume. It was there pointed out that the sort of thinking demanded in the special sciences is not so very different from that with which we are all familiar in common life. Science is more accurate and systematic, it has a broader outlook, and it is free from the imperfections which vitiate the uncritical and fragmentary knowledge which experience of the world yields the unscientific. But, after all, the world is much the same sort of a world to the man of science and to his uncritical neighbor. The latter can, as we have seen, understand what, in general, the former is doing, and can appropriate many of his results. On the other hand, it often happens that the man who has not, with pains and labor, learned to reflect, cannot even see that the philosopher has a genuine problem before him. Thus, the plain man accepts the fact that he has a mind and that it knows the world. That both mental phenomena and physical phenomena should be carefully observed and classified he may be ready to admit. But that the very conceptions of mind and of what it means to know a world are vague and indefinite in the extreme, and stand in need of careful analysis, he does not realize. In other words, he sees that our knowledge needs to be extended and rendered more accurate and reliable, but he does not see that, if we are to think clearly and consciously, all our knowledge needs to be gone over in a different way. In common life it is quite possible to use in the attainment of practical ends knowledge which has not been analyzed and of the full meaning of which we are ignorant. I hope it has become evident in the course of this volume that something closely analogous is true in the field of science. The man of science may measure space and time, and may study the phenomena of the human mind, without even attempting to answer all the questions which may be raised as to what is meant, in the last analysis, by such concepts as space, time, and the mind.