Philosophers in the mid-20th Century and earlier thought that one of the best ways to differentiate between "artificial" things and "natural" things like organisms was that organisms could reproduce themselves.

JohnVonNeumann showed them they were wrong by creating a vast CellularAutomaton which was
	1 capable of UniversalComputation, and
	2 contained a UniversalConstructor and
	3 could therefore reproduce any pattern
	4 including itself....

Since then, SelfReproducingSystem''''''s form a big IntellectualNexus, playing vital roles in ArtificialLife, recursion (like DouglasHofstadter harps on endlessly in GoedelEscherBach), NanoTechnology, EmergentBehavior, the study of ComplexSystems and so forth.

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See: 
* SelfReplication