 Russell’s most important writings relating to these topics include not only his Principles of Mathematics (1903), “Mathematical Logic as Based on the Theory of Types” (1908), and Principia Mathematica (1910, 1912, 1913), but also his earlier Essay on the Foundations of Geometry (1897) and his Introduction to Mathematical Philosophy (1919a), the last of which was written while Russell was serving time in Brixton Prison as a result of his anti-war activities. Coincidentally, it was at roughly this same time that  Ludwig Wittgenstein,  Russell’s most famous pupil, was completing his Tractatus Logico-Philosophicus (1921) while being detained as a prisoner of war at Monte Cassino in Italy during World War I. 