THIS book represents the substance of the Gifford Lectures delivered in Aberdeen in December 1904, June and December 1905, and June 1906. The Lectures were revised by the author, and proofs were read and corrected by him down to the end of Lecture XVII. The MS. of the remaining Lectures was sent to the press about a fortnight before his death, and no proofs of this portion were seen by him, nor had he finally passed the sheets of any part of the book. He intended to go through the whole again carefully, verifying references where he had not already done so. This duty and the correction of Lectures XVIII—XXII have devolved upon me, and, as I have verified the references throughout, the responsibility for any errors that may be found in the text or footnotes, rests with me. A small part of the index had been made, and I have endeavoured to complete it on the same lines.

The author, when he thought that he might not live to finish his task himself, bade me make known his misgiving as to the merits of his work. He was acutely conscious of the difficulties of his subject, more particularly in dealing with Plato's metaphysics, but it may be said that he set down nothing, without taking the most earnest pains to weigh conflicting views, and to form his own judgment by a careful study of all materials that he could collect. One of his last acts was to choose the motto from Xenophanes prefixed to the Lectures.

I wish to acknowledge, with the deepest gratitude, the help given to me, in the correction of the proofs or the preparation of the Memoir, by the Master of Emmanuel, Mr. P. Giles, Mr. L. Whibley, and Mr. T. E. Glover. Mr. Giles was good enough, in addition, to verify references in books to which I had not access. Most sincere thanks are also due to those friends who have entrusted me with letters, or sent notes concerning my husband's life and work.

CAMBRIDGE, March 1908.