WHILE the “Outlines of Physiological Psychology” is not a mere abridgment of Prof. Ladd's larger work, entitled “Elements of Physiological Psychology,” it is cast on the same general lines, and has a similar object in view. At the time of its appearance, we noticed at some length (NATURE, vol. xxxvi. p. 290) the longer and more elaborate treatise. This relieves us of any necessity for saying more than a few words of commendation and criticism of the later and shorter work. In it will be found a careful and accurate description of the results which have been reached through the experimental and physiological study of those neural processes which are regarded as the correlates of mental phenomena. What is known of the neural processes, and what is inferable concerning their psychical concomitants, are clearly and fairly presented; and concerning this part of Prof. Ladd's work we have naught to offer but praise of the impartial consideration of the facts and the judicial presentation of the results. But there are higher and more abstract regions of psychology, where exact correlation of mental processes with neural processes is at present impossible. From the point of view of physiological psychology these should either be left alone, or indicated as regions in which the nature of the correlation, if such exist, is at present unknown. Careful and judicious speculation in this region, if duly presented as such, is fairly admissible. But dogmatic assertions, positive or negative, are an occasion of stumbling to the student, and a serious dereliction of duty on the part of the teacher. When Prof. Ladd says that “a thorough analysis of mental life discloses other forms of activity than those properly called sensory-motor,” he is well within his province; but when he proceeds to lay it down “that these forms not only do not find their full explanation in the changing state of the brain, but are not even conceivable as correlated with such states,” he is building too confidently on his ignorance. It is noteworthy that, where our ignorance concerns a conclusion which he rejects, the conclusion is said to be “inconceivable”; but where it concerns a conclusion which he accepts (as in the case of the causal connection between the mind and the body as distinct entities), it is called a “mystery,” and we are gravely admonished that “because of its mystery it is no less to be acknowledged as a fact.” What is here spoken of as a “fact” is one solution out of several of one of the most difficult philosophical problems. Just where the scientific spirit, as opposed to that which is the outcome of other considerations, demands increased caution and modesty, since the grounds of perceptual verification are far distant, and since investigators of admitted knowledge, insight, and fairmindedness have reached other conclusions, Prof. Ladd assumes a dogmatic tone, which seems to us a serious error in an otherwise excellent work.