The philosophy of Idealism, revived in eighteenth-century Europe by George Berkeley, argued against philosophical materialism by maintaining that Reality is a creation of the Mind. Despite its flourishing under the leadership of Hegel, Fichte, Schopenhauer, and Schelling, Idealism had definitely fallen into decline late in the nineteenth century and early in the twentieth. May Sinclair, the writer of many popular but philosophically provocative novels and part-time World War I ambulance corps-person, was an unlikely one to take up the torch of the old school and try to revive it yet again for the twentieth century. Most notably, in this treatise she tried to formulate a union of Idealistic Monism with the New Realism as epitomized by Bertrand Russell and his mathematics-based philosophy. How successful she was is a matter for the reader to judge, but this volume and its sequel (The New Idealism) provide fascinating insights into the mind of Sinclair herself, helping the twenty-first century reader better to understand this highly significant author as she wrote in the very midst of the Great War.