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later than 1703. Berkeley gives no direct date of the birth of his theories, but it can be supplied approximately from his "Commonplace Book." In this book, according to Fraser, he "seems to have set down . . . stray thoughts which occurred to him in the course of his mathematical and metaphysical studies at Trinity College, Dublin. These common-places seem to have been formed gradually, apparently in 1705 and some following years. . . . Considerable portions imply that he was at the time maturing his thoughts with a view to the publication of the Essay on Vision and the Principles of Human Knowledge but the form which the projected work (or works) was to take does not appear to have been finally settled in his mind."^[1] It is not possible to compare definite dates here. We can only say that in 1703 Collier was convinced of his theory, and that in 1705 Berkeley was testing his doctrine by applying it to all branches of knowledge. If these dates bear any weight, we may conclude that the two men, while they were both under the age of twenty-five and while they were still continuing their college studies, independently conceived this new theory of matter. From the references in the letters written in the few years following the publication of the "Clavis," it is evident that in the meantime Collier had become acquainted with some one book of Berkeley; and the further reference in the "Speci-