1. Several years have elapsed since I first conceived the design of working out the problems of rational psychology, or of investigating the essence and faculties of the human soul and internal senses; but hitherto the extreme difficulty of the subject, and the vast amount of preliminary knowledge which is requisite, have warned me against treading prematurely in this direction; for many things must first be clearly expounded and arranged in order. Certain it is, that those who are but superficially informed in the elements of the sciences, or even well acquainted with one or two branches, and who notwithstanding dare to enter upon this field, because of their ignorance of many and sometimes of the leading means, will but exert an unprofitable subtlety for the most part on empty questions, and only discuss ingenious trifles; and according to their natural endowments which are enlightened, or as it often happens, obscured by a few sciences, they will involve an important subject in thorny discussions, which can have no effect upon the brains, but to occasion disputes which a judge will in vain try to settle, on account of the trivial nature of the matter upon which the acumen and subtlety are expended. The present subject, however, is too important to be so treated; it is no arena for foils, but for naked weapons. To complete the single science of the soul, all the sciences are required that the world has ever eliminated or developed. If any shall undertake the task with less than all, it must be with the certainty of discovering in the end that he is destitute of the instruments, and unequal to the toils of so vast a work. The points which he requires, but of which unhappily he is ignorant, he must perforce obtain from himself or produce by the keenness of his own mind, that is to say, he must use his imagination to supply the place of real knowledge; and how prone to error the imagination is if left to her own guidance, without the companionship of experience and the precepts of a true philosopher, is perfectly well known to even the least experienced. Let the reader only make the attempt for a short distance, and he will soon find himself retreating with his standards, and obliged to bring up fresh supplies, so that he may with increased forces make attempts of this kind.
