Whitehead spent the first 25 years of his teaching career at Trinity College, Cambridge. Whitehead and Bertrand Russell (1872–1970) distanced themselves from each other after Russell became a pacifist during World War I and Whitehead’s son was killed in that war. Whitehead taught at the University of London and began publishing works on philosophy of science, such as Principles of Natural Knowledge (1919), The Concept of Nature (1920), and The Principle of Relativity (1922). His most important work as a process philosopher was Process and Reality (1927–1928), which was published after he moved to the United States to accept a position at Harvard.