This way, a possibility is found for quantizing classically chaotic systems, which lately has been mentally prepared already by Poincaré. Namely, he conjectured that the unstable periodic orbits of low-dimensional non-integrable systems (like the three body problem) play a special role in the understanding of their irregular motion.

Father of this pioneering development in quantum mechanics, which puts the old quantum theory in a new light by virtue of a renaissance of the semiclassical theory, is Martin Gutzwiller.