same author when he has made for himself a name, when he knows something of the feeling of the public as to his powers, and when also he has learned to distinguish the qualities he possesses—to see where he is strong and where weak—will have an air of strength and firmness not due, or only partially due, to any real growth of his mental powers. But, as I have said, and as experience has repeatedly shown, it is in opinions formed as to the diminution of mental power that the world is most apt to be deceived. How commonly the remark is heard that So-and-so has written himself out, or Such-a-one is not the man he was, when in reality, as those know who are intimate with the author so summarily dismissed, the deterioration, justly enough noted, is due to circumstances in no way connected with mental capacity! The author who has succeeded in establishing a reputation may not have (nay, very commonly has not) the same reason for exerting his powers to the full, as he had when he was making his reputation. He may have less leisure, more company, new sources of distraction, and so on. The earlier work, his chef-d'oeuvre, let us say, may have been produced at one great effort, no other subject being allowed to occupy his attention until the masterpiece had been completed—the later and inferior work, hastily accepted as evidence that the author's mind no longer preserves its wonted powers, may have been written hurriedly and piecemeal, and subjected to no jealous revision before passing through the press.
Here I have taken literary work as affording typical instances. But similar misapprehensions are common in other departments of mental work. For example, it is related that Newton, long before he was an old man, said of himself that he could no longer follow the reasoning of his own "Principia," and this has commonly been accepted as evidence that his mind had lost power. The conclusion is an altogether unsafe one, as every mathematician knows. It would have been a truly wonderful circumstance if Newton had been able, even only ten or twelve years after his magnum opus was completed, to follow its reasoning with satisfaction to his own mind—that is, with the feeling that he still had that grasp of the subject which he had possessed when, after long concentration of his thoughts upon it, he was engaged in the task of exhibiting a summary of his reasoning (for the "Principia" is scarcely more).
I can give more than one instance, in my own experience, of this seeming loss of mastery over a mathematical subject, while in reality the mind has certainly not deteriorated in its power of dealing with subjects of that particular kind. I will content myself with one. It happened that in 1869 I had occasion to examine a mathematical subject of no very great difficulty, but involving many associated relations, and requiring therefore a considerable amount of close attention. At that time I had made myself master, I think I may say without conceit, of that particular subject in all its details. Recently, I had