Cantor was often misunderstood, both by his contemporaries and by later historians. Leopold Kronecker strongly opposed Cantor's work and called him a scientific fraud, a renegade, and a “corrupter of youth.” In contrast, Bertrand Russell considered Cantor one of the greatest thinkers of the nineteenth century. David Hilbert praised Cantor's work and believed it had opened new possibilities in mathematics. Henri Poincaré and others disagreed. They considered set theory and Cantor's theory of transfinite numbers a serious problem in mathematics that would eventually need to be corrected. Georg Cantor (1845–1918), the founder of transfinite set theory, was one of the most influential and controversial mathematicians in history. In the late nineteenth century, his work on continuity and infinity led him to develop new ideas that differed greatly from the traditional understanding of infinity in mathematics.