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. LONDON, _September, 1917_ CONTENTS _Chapter_ _Page_ I. _Mysticism and Logic_ 1 II. _The Place of Science in a Liberal Education_ 33 III. _A Free Man's Worship_ 46 IV. _The Study of Mathematics_ 58 V. _Mathematics and the Metaphysicians_ 74 VI. _On Scientific Method in Philosophy_ 97 VII. _The Ultimate Constituents of Matter_ 125 VIII. _The Relation of Sense-data to Physics_ 145 IX. _On the Notion of Cause_ 180 X. _Knowledge by Acquaintance and Knowledge by Description_ 209 MYSTICISM AND LOGIC AND OTHER ESSAYS I MYSTICISM AND LOGIC Metaphysics, or the attempt to conceive the world as a whole by means of thought, has been developed, from the first, by the union and conflict of two very different human impulses, the one urging men towards mysticism, the other urging them towards science. Some men have achieved greatness through one of these impulses alone, others through the other alone: in Hume, for example, the scientific impulse reigns quite unchecked, while in Blake a strong hostility to science co-exists with profound mystic insight. But the greatest men who have been philosophers have felt the need both of science and of mysticism: the attempt to harmonise the two was what made their life, and what always must, for all its arduous uncertainty, make philosophy, to some minds, a greater thing than either science or religion.