  For years, the biggest unspeakable in quantum theory has been why quantum
theory and what is quantum theory telling us about the world. Recent efforts
are unveiling a surprisingly simple answer. Here we show that two
characteristic limits of quantum theory, the maximum violations of
Clauser-Horne-Shimony-Holt and Klyachko-Can-Binicio\u{g}lu-Shumovsky
inequalities, are enforced by a simple principle. The effectiveness of this
principle suggests that non-realism is the key that explains why quantum
theory.
