" We have seen recursion in the grammars of languages, we have seen recursive geometrical trees, which grow upwards forever, and we have seen one way in which recursion enters the theory of solid state physics. Now we are going to see yet another way in which the whole world is built out of recursion. This has to do with the structure of elementary particles: electrons, protons, neutrons, and the tiny quanta of electromagnetic radiation called "photons". We are going to see that particles are - in a certain sense which can only be defined rigorously in relativistic quantum mechanics- nested inside each other in a way which can be described recursively, perhaps even by some sort of "grammar". "

― Douglas R. Hofstadter , Gödel, Escher, Bach: An Eternal Golden Braid