I am far from thinking that the more recondite specimens of analysis in this work are always successful, or that the author has not left something to be corrected as well as much to be completed by his successors. The completion has been especially the work of two distinguished thinkers in the present {xix} generation, Professor Bain and Mr. Herbert Spencer; in the writings of both of whom, the Association Psychology has reached a still higher development. The former of these has favoured me with his invaluable collaboration in annotating the present work. In the annotations it has been our object not only to illustrate and enforce, but to criticise, where criticism seemed called for. What there is in the work that seems to need correction, arises chiefly from two causes. First, the imperfection of physiological science at the time at which it was written, and the much greater knowledge since acquired of the functions of our nervous organism and their relations with the mental operations. Secondly, an opening was made for some mistakes, and occasional insufficiency of analysis, by a mental quality which the author exhibits not unfrequently in his speculations, though as a practical thinker both on public and on private matters it was quite otherwise; a certain impatience of detail. The bent of his mind was towards that, in which also his greatest strength lay; in seizing the larger features of a subject--the commanding laws which govern and connect many phenomena. Having reached these, he sometimes gives himself up to the current of thoughts which those comprehensive laws suggest, not stopping to guard himself carefully in the minutiæ of their application, nor devoting much of his thoughts to anticipating all the objections that {xx} could be made, though the necessity of replying to some of them might have led him to detect imperfections in his analyses. From this cause (as it appears to me), he has occasionally gone further in the pursuit of simplification, and in the reduction of the more recondite mental phenomena to the more elementary, than I am able to follow him; and has left some of his opinions open to objections, which he has not afforded the means of answering. When this appeared to Mr. Bain or myself to be the case, we have made such attempts as we were able to place the matter in a clearer light; and one or other, or both, have supplied what our own investigations or those of others have provided, towards correcting any shortcomings in the theory.