 This entry discusses philosophical idealism as a movement chiefly in the eighteenth and nineteenth centuries, although anticipated by certain aspects of seventeenth century philosophy and continuing into the twentieth century. It revises the standard distinction between epistemological idealism, the view that the contents of human knowledge are ineluctably determined by the structure of human thought, and ontological idealism, the view that epistemological idealism delivers truth because reality itself is a form of thought and human thought participates in it, in favor of a distinction earlier suggested by A.C. Ewing, between epistemological and metaphysical arguments for idealism as itself a metaphysical position. After discussing precursors, the entry focuses on the eighteenth-century versions of idealism due to Berkeley, Hume, and Kant, the nineteenth-century movements of German idealism and subsequently British and American idealism, and then concludes with an examination of the attack upon idealism by Moore and Russell and the late defense of idealism by Brand Blanshard. 