DR. KROELL'S work might be judged from two very different points of view. As a popular and generally intelligible account of the present state of our knowledge as to the localisation of function in the brain and the stages of cerebral development, some of his chapters may be highly commended for their clearness and accuracy. As a “psychological sketch” of human life and thought, written with the avowed object of establishing the materialist position, the book is an unqualified failure. Dr. Kroell brings out materialism in his results simply because he has put it into his premisses. He is content to assume, the first principles of mechanical physics, hot, as a real physicist might do, as working hypothesis, but as unquestionable and ultimate truth. Moreover, he states even those principles in an unsatisfactory way. The difficulties which beset the problem of the relation of matter and energy are ignored by the convenient device of asserting that each is one aspect of a double reality which the author calls “Kraft-stoff”; unfortunately he omits to tell us how “Kraft-stoff” is to be measured. He assumes, with equal recklessness, that all energy is kinetic (p. 28) and (pp. 30, 31) that the phenomena of life must be capable of being adequately described in terms of rotatory motion. Dr. Kroell's psychology is of the same type. He can see no difference in principle between the photo-chemical changes produced on the retina by a light-stimulus and the transformation of molecular motion into consciousness which, on his theory, take place in the cortex. The “picture in the brain” is a reality of the same order as the “picture on the retina.” That neither “picture,” as distinct from its physical conditions, exists except for the eye of an observer does not occur to him. A sensation (p. 70) is actually said to be a cerebral process which has become “conscious of itself,” though, of course, our own cerebral processes are in point of fact precisely that of which we are never directly conscious. The sensation as a mental state is confused with its own cortical concomitants and baptised by the name of “picture in the cortex” (p. 98); and the unmeaning question has then to be discussed how it comes about that the “picture in the cortex” is “referred away outside” to the periphery of the body or to a spot in the external world. The questions of animal psychology, which all serious students of the subject know to require the most cautious handling, are settled by Dr. Kroell in the same spirit of jaunty confidence. For instance (p. 125), the higher animals have memory-images. This is roundly affirmed without evidence, apparently in utter ignorance of the actual experimental work which has been done in the study of the animal mind and the much more guarded conclusions to which that work points. Animals (ib.) have “concepts,” though no word is said as to the evidence which has satisfied the author upon this thorny and debated subject. These are but a few specimens of the confusions of thought and reckless assertions with which the book abounds; they should be enough, however, to indicate the worth of an argument for materialism founded on such premisses. Psychologists have no right to quarrel with physiologists and medical men for not being themselves psychologists; they surely have a right to expect that psychology, as much as any other science, should be protected against the dogmatism of outsiders who disdain to make themselves acquainted with its problems and methods. No knowledge of physiology can give its possessor the right to dogmatise a priori in physics and in psychology. Der Aufbau der Menschlichen Seele; Fine Psychologische Skizze. Von Dr. H. Kroell. Pp. v + 392. (Leipzig: Engelmann, 1900.)