In the second half of the nineteenth century, logic awoke from a sleep of two millennia: people realised that Aristotle, with his syllogisms, had yet to have the last word on the subject. Powerful and flexible systems were developed by the English mathematician George Boole as well as by the German logician Gottlob Frege. Soon, however, logic found itself entangled in self-reference (a statement which refers to itself or its own referent). Curiously, the self-reference debate was contemporary with the discovery of quantum theory, and Austrian logician Kurt Gödel's theorem of incompleteness was proved at about the same time as the uncertainty principle, with similar effects.