 Even more importantly, the questions left open by Cantor and emphasized by Hilbert in his first problem of 1900 caused heated debate. At the International Congress of Mathematicians at Heidelberg, 1904, Gyula (Julius) König proposed a very detailed proof that the cardinality of the continuum cannot be any of Cantor’s alephs. His proof was only flawed because he had relied on a result previously “proven” by Felix Bernstein, a student of Cantor and Hilbert. It took some months for Felix Hausdorff to identify the flaw and correct it by properly stating the special conditions under which Bernstein’s result was valid (see Hausdorff 2001, vol. 1). Once thus corrected, König’s theorem became one of the very few results restricting the possible solutions of the continuum problem, implying, e.g., that \(\textit{card}(\mathbf{R})\) is not equal to \(\aleph_{\omega}\). Meanwhile, Zermelo was able to present a proof that every set can be well-ordered, using the Axiom of Choice [1904]. During the following year, prominent mathematicians in Germany, France, Italy and England discussed the Axiom of Choice and its acceptability.