The following essays have been written and published at various times, and my thanks are due to the previous publishers for the permission to reprint them. The essay on "Mysticism and Logic" appeared in the Hibbert Journal for July, 1914. "The Place of Science in a Liberal Education" appeared in two numbers of The New Statesman, May 24 and 31, 1913. "The Free Man's Worship" and "The Study of Mathematics" were included in a former collection (now out of print), Philosophical Essays, also published by Messrs. Longmans, Green & Co. Both were written in 1902; the first appeared originally in the Independent Review for 1903, the second in the New Quarterly, November, 1907. In theoretical Ethics, the position advocated in "The Free Man's Worship" is not quite identical with that which I hold now: I feel less convinced than I did then of the objectivity of good and evil. But the general attitude towards life which is suggested in that essay still seems to me, in the main, the one which must be adopted in times of stress and difficulty by those who have no dogmatic religious beliefs, if inward defeat is to be avoided. The essay on "Mathematics and the Metaphysicians" was written in 1901, and appeared in an American magazine, The International Monthly, under the title "Recent Work in the Philosophy of Mathematics." Some points in this essay require modification in view of later work. These are indicated in footnotes. Its tone is partly explained by the fact that the editor begged me to make the article "as romantic as possible." All the above essays are entirely popular, but those that follow are somewhat more technical. "On Scientific Method in Philosophy" was the Herbert Spencer lecture at Oxford in 1914, and was published by the Clarendon Press, which has kindly allowed me to include it in this collection. "The Ultimate Constituents of Matter" was an address to the Manchester Philosophical Society, early in 1915, and was published in the Monist in July of that year. The essay on "The Relation of Sense-data to Physics" was written in January, 1914, and first appeared in No. 4 of that year's volume of Scientia, an International Review of Scientific Synthesis, edited by M. Eugenio Rignano, published monthly by Messrs. Williams and Norgate, London, Nicola Zanichelli, Bologna, and Félix Alcan, Paris. The essay "On the Notion of Cause" was the presidential address to the Aristotelian Society in November, 1912, and was published in their Proceedings for 1912-13. "Knowledge by Acquaintance and Knowledge by Description" was also a paper read before the Aristotelian Society, and published in their Proceedings for 1910-11.