A starting point would be a system that does not require the use of a limited set of pre-defined operations to transform from one state to another state via the interpretation of a set of pre-existing instructions. This rules out any digital system entirely.
But what _would_ qualify? The point being made is that your definition is so constricting as to be useless. Nothing (sans perhaps true physical limit-conditions, like black-holes) would be a black box under your definition.
