Ontology is the baseline. [All definitions are made in terms of other definitions in what only ever arrives at circularly tautological uncertainty. An arbitrary starting point must be chosen from inside the hyper-inflating self-referential systems of language and logic.]
What does a “whole” or “complete” (if feeling adventurous, add “consistent” or “decidable”) system look like?
There is no external property or reality. Everything is internal to the referential system and all evolution, innovation and developmental ratchets are the self-inflected introspection of that system. Complex systems drift through entropy into sophistication and logical depth; implying indefinitely-extensible, context-agnostic hyper-inflation; the interior surface of an asymptotic abstraction on the arc towards singularity and a closure only ever demonstrable as a function of its own absence.
Think of two systems as measure and record of difference or information of self (inside) and world (outside) that each contain the other, mirrored in and as one system that contains itself. It is only possible if the system self-containment is globally-distributed. This is only possible if self-containment is a manifest systems property of recursive self-negation.