Wittgenstein finished part I of the "Philosophical Investigations" in the spring of 1945. From 1946 to 1949 he worked on the philosophy of psychology almost without interruption. The present two-volume work comprises many of his writings over this period. Though some of these remarks appeared in volume 2 of the "Investigations" and others in "Zettel," the great majority, although of excellent quality, remained uncollected until 1980.