It was a feature peculiar to the late Sir W. Hamilton's character to avoid writing on any subject, with a view to publicity, until he had mastered everything extant which had previously been written upon it. It is said that when Hamilton, on its first appearance, read Whately's Rhetoric, he accused the author of plagiarism. A mutual friend suggested that Whately most probably had never even heard of the sources supposed to have been laid under contribution. Then why, asked Hamilton, did he presume to write on the subject at all? Now it is just because Hamilton himself was so punctilious in this respect that his researches are of great value to the psychologist. The investigator who masters these, and builds on them, may feel confident that he does not break with the past, but that his conclusions legitimately spring, as all such conclusions should do, positively, out of the capital of knowledge which our forefathers have amassed, and negatively, out of the errors they have committed.