The volume contains four essays. The first, on the _Nature of Life_, deals with the speciality of organic phenomena, as distinguished from the inorganic. It sets forth the physiological principles which Psychology must incessantly invoke. In the course of the exposition I have incorporated several passages from four articles on Mr. Darwin’s hypotheses, contributed to the _Fortnightly Review_ during the year 1868. I have also suggested a modification of the hypothesis of Natural Selection, by extending to the _tissues and organs_ that principle of competition which Mr. Darwin has so luminously applied to _organisms_. Should this generalization of the “struggle for existence” be accepted, it will answer many of the hitherto unanswerable objections. The second essay is on the _Nervous Mechanism_, setting forth what is known and what is inferred respecting the structure and properties of that all-important system. If the sceptical and revolutionary attitude, in presence of opinions currently held to be established truths, surprises or pains the reader unprepared for such doubts, I can only ask him to submit my statements to a similar scepticism, and confront them with the ascertained evidence. After many years of laborious investigation and meditation, the conclusion has slowly forced itself upon me, that on this subject there is a “false persuasion of knowledge” very fatal in its influence, because unhesitatingly adopted as the ground of speculation both in Pathology and Psychology. This persuasion is sustained because few are aware how much of what passes for observation is in reality sheer hypothesis. I have had to point out the great extent to which Imaginary Anatomy has been unsuspectingly accepted; and hope to have done something towards raising a rational misgiving in the student’s mind respecting “the superstition of the nerve-cell”--a superstition which I freely confess to have shared in for many years. The third essay treats of _Animal Automatism_. Here the constant insistance on the biological point of view, while it causes a rejection of the mechanical theory, admits the fullest recognition of all the mechanical relations involved in animal movements, and thus endeavors to reconcile the contending schools. In this essay I have also attempted a psychological solution of that much-debated question--the relation between Body and Mind. This solution explains why physical and mental phenomena must necessarily present to our apprehension such profoundly diverse characters; and shows that Materialism, in attempting to deduce the mental from the physical, puts into the conclusion what the very terms have excluded from the premises; whereas, on the hypothesis of a physical process being only the objective aspect of a mental process, the attempt to interpret the one by the other is as legitimate as the solution of a geometrical problem by algebra.