Physiology is co-extensive with organic nature. Organic nature is wholly composed of individuals, comprising the two great kingdoms of plants and animals. A unity of structure pervades the whole of this wide field of nature; and this unity is a great principle, applicable to the determination of truth in the investigation of this part of knowledge. Every individual in organic nature is a system made up of reciprocally dependent and connected parts. The objects of investigation in physiology are phenomena, organs, and principles. The study of phenomena stands first in order; but while it must essentially be first cultivated and advanced, in the ulterior stages of its progress it gains continually fresh additions from the progress made in the knowledge of organs and principles. That phenomena attract attention before organs, is manifest on the slightest consideration. Thus the phenomena of locomotion were familiar to mankind long before the part taken by the muscular flesh in locomotion was discovered. To this moment it is far more certain that absorption takes place throughout the animal body, than what the organs are by which that office is performed. And it would be easy to multiply examples of the same kind, not-withstanding that there are some phenomena of the human body—such as those connected with the sense of sight, the sense of hearing, and other senses—the organs concerned in which must have been known, in a general manner, almost as soon as the earliest phenomena in which they are concerned. Principles, in their larger sense, take their place subsequently to the study of organs; yet, as referring to the more common genera of phenomena, these must also have had their rise almost coeval with the observation of phenomena. Thus the grouping of colours, sounds, smells, and tastes together, under the name of qualities derived from sense, must have been a very early and universal generalization. Nevertheless, it will, I think, be conceded, after these examples, that the study of phenomena is of a more elementary character in physiology, than the study of organs and principles; and, therefore, in the difficult parts of any physiological subject, that more progress is likely to be made by the study of phenomena, than by the study of organs and principles. But before proceeding further, it may be desirable to give some examples of physiological phenomena:—the alternation of sleep and waking; of hunger and satiety; thirst; the effect of drink; breathing; the exercise of the senses, and trains of thought; the various kinds of locomotion, walking, running, leaping, dancing. Here a question naturally arises—if trains of thought be physiological phenomena, does not all human knowledge fall within the definition of physiological phenomena? If the human race were not yet called into being, neither would human knowledge, it is true, have any existence in the world. And, it is doubtless true, under one point of view, that all that man has discovered; all that he has recorded; all the changes which he has made upon the earth since his first creation—are the effects of his physiological nature. But to place all knowledge under the head of physiology would be to defeat the very end of methodical arrangement, to which the progress of knowledge is so largely indebted. Nor is it difficult to mark out at least the general character of the boundaries within which physiology, in the largest sense in which it is convenient to accept it, should be circumscribed. Let us take as an example man's susceptibility of locomotion. It is a sufficient illustration of the physiology of locomotion to point out, that every man without any extraordinary effort learns to walk, run, hop, leap, climb; but there is at least a manifest convenience in separating such more difficult acquisitions as dancing, skating, writing, from the order of physiological phenomena, and placing each in a department by itself, as subject to its own rules. So also it is at least a convenience to consider painting and music as separate departments of study, and not merely as physiological phenomena, falling under the senses of sight and of hearing. It may be supposed to be a matter of the like convenience, to separate from physiology all the phenomena which enter into what are commonly called trains of thought; that is nearly all that comes under the head of psychology, in its most appropriate extent of signification. But several objections will readily occur to such a mutilation of physiology. In particular, it is objectionable, because, as was already hinted, the phenomenal departments of physiology, though the first to take a start, are often much augmented by the subsequent study of the organs concerned; and, more so that, since psychology, disjoined from physiology, and limited to one mode of culture, namely, by reflexion on the subjects of consciousness, were psychology thrown out from physiology, the probable advantages from the study of the organs concerned in the mental processes, and the other modes of culture, admissible in physiological enquiry, would be lost. If it be said that psychology proper rejects all evidence, except the evidence of consciousness, on no other ground, but because of the uncertainty of every other source of evidence—the answer is, that in those sciences which have made most progress, possibility, probability, and moral certainty have always been admitted as sufficient interim grounds for the prosecution of such inquiries as have finally, though at first leading to inexact conclusions, opened the way to the attainment of the most important truths; and that psychology, by the over-rigidness of its rules of investigation, has plainly fallen behind sciences, in advance of which it at one time stood in its progress.