He now started an intensive study of the foundations of mathematics at Trinity. In 1897, he wrote "An Essay on the Foundations of Geometry" (submitted at the Fellowship Examination of Trinity College) which discussed the Cayley–Klein metrics used for non-Euclidean geometry. He attended the First International Congress of Philosophy in Paris in 1900 where he met Giuseppe Peano and Alessandro Padoa. The Italians had responded to Georg Cantor, making a science of set theory; they gave Russell their literature including the "Formulario mathematico". Russell was impressed by the precision of Peano's arguments at the Congress, read the literature upon returning to England, and came upon Russell's paradox. In 1903 he published "The Principles of Mathematics", a work on foundations of mathematics. It advanced a thesis of logicism, that mathematics and logic are one and the same.