Georg Cantor
Notes
Georg Cantor (1845–1918), German mathematician who founded set theory and showed that infinities come in different sizes: the real numbers cannot be put into one-to-one correspondence with the natural numbers, and there is no largest infinity. His work was attacked in his lifetime as philosophically and theologically dangerous, and he spent his later years in and out of sanatoria. It is the reason the claim that an actual infinite "cannot exist" (Kalam Cosmological Argument, Hilbert's Hotel) cannot be made on mathematical grounds: the mathematics of actual infinities is consistent, and has been for a century and a half.
Wikipedia: https://en.wikipedia.org/wiki/Georg_Cantor