It was at this time that he became interested in the foundations of mathematics , particularly after reading Bertrand Russell 's The Principles of Mathematics ( 1903 ) , and Gottlob Frege 's Grundgesetze der Arithmetik , vol .
During his research he became interested in the foundations of mathematics , particularly after reading Bertrand Russell 's Principles of Mathematics and Gottlob Frege 's Grundgesetze .