To begin with, the attitude of the author to his subject is that of a professional scientist to his specialty. "I have kept close," he says, "to the point of view of natural science throughout the book. Every natural science assumes certain data uncritically, and declines to challenge the elements between which its own 'laws' obtain, and from which its own deductions are carried on.... This book, assuming that thoughts and feelings exist, and are vehicles of knowledge, thereupon contends that Psychology, when she has ascertained the empirical correlation of the various sorts of thought or feeling with definite conditions of the brain, can go no farther—can go no farther that is as a natural science. If she goes farther she becomes metaphysical." This position does not carry with it the condemnation of all matters metaphysical, but simply excludes them from Psychology; nor does this independence place Psychology in a position unrelated to other sciences. Such relation is a cardinal fact in the mental world, and nowhere is it more necessary to bear in mind that the division of the sciences is largely an expression of the lines of men's interests and the inevitable specialisation of knowledge. Those forms of adaptations of means to ends which we study as forms of psychic action, while theoretically distinguishable from other modes of action, in fact, often resemble them; in other words, "the boundary line of the mental faculty is certainly vague. It is better not to be pedantic, but to let the science be as vague as the subject," and include all facts, whether they are usually called physiological, or biological, or not, that shed light on the main problems dealt with. This conception accordingly views mind as distinctly related to and an essential part of its environment; it views mental phenomena as infinitely varied, as most intricately conditioned by and in turn conditioning other natural phenomena. For the complete survey of its domain, it calls upon experiment, observation, introspection, comparison, analysis, hypothesis, deduction, each properly controlled by the others, and limited by community of purpose to a firm foundation of fact. It is true that in the more intricate problems, those with the smallest connection with sensation and the largest with inference and analysis, the author will be regarded as more metaphysical than psychological and plainly admits his fault; it is true that the personal leanings of the author lead him to lengthy discussions of these more intricate points, but none the less the positive, broad, and evolutionary spirit that dominates the general view of the subject leaves a clear impress of vitality, progress, and interest on every page.