In contradistinction to Professor Mach I maintained that our scientific terms, although abstract concepts and things of thought, or noumena, are after all descriptive of actual facts; they are symbols representing features of reality. Motions, i. e., that which is meant by the term motion, is a reality, and what the chemist calls atoms is a definite quality of certain facts of experience. Atoms are not things in themselves, as the name seems to suggest, but rather proportional relations conveniently so expressed as if they were ultimate units or concrete little bodies of a definite mass or weight. What atoms are, aside from representing the proportions in which elements combine, we do not know. We may define “atom” as the minimal weight in which an element enters into chemical combinations, but such atoms have never been an object of observation. For aught we know, they may as little be discrete bodies as a curve consists of discrete straight lines, which, as such, would be unobservable only because infinitesimally small. The infinitely small straight line into which a curve is analysed by mathematicians is a fiction, wisely devised for calculating the path of the curve. This fiction is as Professor Mach says, an artifice only, not a reality, or as I say, an allegoric expression to characterise not whole concrete realities, but certain features of reality in their abstractness. Scientific terms are comparable to myths that contain deep religious truths. The fiction of the myth is only the vehicle of its meaning. The naked meaning in its abstract purity may be difficult to grasp. Thus our imagination steps in and completes the picture so as to render it concrete and easily thinkable. Now, when several months ago I met Professor Mach at Prague, our conversation naturally touched upon the problems which had formed the subject of our discussion. Professor Mach assented to my speaking of scientific terms as abstracts. That, accordingly, must be considered as the point of agreement. But when I proposed that the term sensation also was according to my terminology an abstract term representing one feature of reality only and excluding other features, Professor Mach took exception to it, saying that he understands by sensation reality itself. Very well then, this is the difference; and this difference is after all a difference of terms only. I understand by sensation the psychical feature of the data of experience only, to the exclusion of what may be called its physical aspect. Sensation accordingly, as I use the term, is not the whole of the given reality but only one of its qualities. If, as Professor Mach uses the term, sensation is another name for reality, the main difference between our views appears to be removed. P. CARUS. BOOK REVIEWS.