On his mother's side, Hayek was second cousin to the philosopher Ludwig Wittgenstein. His mother often played with Wittgenstein's sisters, and had known Ludwig well. As a result of their family relationship, Hayek became one of the first to read Wittgenstein's Tractatus Logico-Philosophicus when the book was published in its original German edition in 1921. Although Hayek met Wittgenstein on only a few occasions, Hayek said that Wittgenstein's philosophy and methods of analysis had a profound influence on his own life and thought. In his later years, Hayek recalled a discussion of philosophy with Wittgenstein, when both were officers during World War I. After Wittgenstein's death, Hayek had intended to write a biography of Wittgenstein and worked on collecting family materials; and he later assisted biographers of Wittgenstein.