Carl Ludwig Siegel

“I am afraid that mathematics will perish by the end of this century if the present trend for senseless abstractin — as I call it: theory of the empty set — cannot be blocked up.”

“Ours, according to Leibnitz, is the best of all possible worlds, and the laws of nature can therefore be described in terms of extremal principles. Thus, arising from corresponding variational problems, the differential equations of mechanics have invariance properties relative to certain groups of coordinate transformations.”