“Unser Wissen wird nie vollendet, ist und bleibt Stückwerk; dessen Ergänzung das Streben und Hoffen der forschenden Denker bleiben wird für alle Zeit.”--RADENHAUSEN, _Osiris_. “Our nimble souls Can spin an insubstantial universe Suiting our mood, and call it possible, Sooner than see one grain with eye exact, And give strict record of it.” GEORGE ELIOT, _The Spanish Gypsy_. “If we compare the teachings of our books with what Nature is constantly showing, we find there is no agreement between those two sources of learning.”--BROWN SÉQUARD. THE NERVOUS MECHANISM. CHAPTER I. SURVEY OF THE SYSTEM. 1. Our knowledge of mental processes is derived from reflection on our personal experiences, combined with inferences from our observation of other men and animals, under similar conditions. The processes are complex and variable; so complex and variable, that knowledge of their component factors can only be gained through long tentative study, aided by fortunate circumstances which present these factors separately, or at any rate in such marked predominance as to fix attention. This subjective analysis of the _processes_ has to be supplemented by, and confirmed by an objective analysis of, the _conditions_, external and internal: the facts of Feeling have to be traced to facts of Physiology, which will exhibit that Physical Basis of Mind so earnestly sought by the inquirer. Both the subjective and the objective analysis are at present in a very imperfect state. Although there is much confident assertion and “false persuasion of knowledge” in both regions, there is, unhappily, little that can be seriously accepted as demonstrated. In the present volume we shall concern ourselves almost exclusively with the objective analysis, and do our utmost to mark what is mere inference from what is verified observation. It is only by Observation that facts can be _settled_; however Analogy and Inference may suggest where the truth may lie, they are finger-posts, not goals. At the best they only tell us what Observation _would_ reveal could the processes be submitted to Sense.