I hold that the propositions embodied in natural science are not derived by any definite rule from the data of experience, and that they can neither be verified nor falsified by experience according to any definite rule.

An outstanding teacher; two of his pupils won Nobel Prizes. He emigrated to Germany working at the Kaiser Wilhelm Institute and then to England working at the University of Manchester. He pioneered the theory of fiber diffraction analysis in 1921 and the dislocation theory of plastic deformation of ductile metals and other materials in 1934.