The idea that quantum physics might play an important role in living systems has long been considered by physicists. Niels Bohr for example believed that his principle of complementarity extended into both biology and psychology, while Erwin Schrödinger wrote in his 1944 book What is Life? of a "quantum theory of biology" that saw genetic mutations in terms of quantum leaps. In his 1989 book The Emperor's New Mind, Roger Penrose hypothesized that quantum mechanics plays an essential role in human consciousness. His 1994 follow-up book Shadows of the Mind speculated that these quantum processes take place in microtubules inside neurons.