Along with this rigorization of mathematics there appeared a tendency toward abstract generalization, a process whith has became very pronounced in present-day mathematics. Perhaps the German mathematician Georg Bernhard Riemann (1826-1866) influenced this feature of modern mathematics more than any other nineteenth-century mathematician. His doctoral dissertation of 1851 led to the concept of Riemann surfaces, which, in turn, introduced so-called topological considerations into analysis. Riemann clarified the concept of integrability by the definition of what we now know as the Riemann integral, which led, in the twentieth century, to the more general Lebesgue integral, and thence to further generalizations of the integral. Riemann’s famous probationary lecture of 1854 on the hypotheses which lie at the foundation of geometry generalized the idea of space and led, in more recent times, to the extensive and important theory of abstract spaces. Riemann died of tuberculosis when only forty, but he left to the mathematical world, in his small collection of published papers, a singularly rich legacy of ideas not yet exhausted by later mathematicians.