The question came to me when wondering how to begin defining addition.
On that question I came across this:
I noticed they reasonably begin by defining the natural numbers.
However I also noticed they resort to other already established systems from where to draw those definitions, be it set theory or the lambda calculus.
So is there a more foundational theory, within which all such systems, eg. addition, the natural numbers, the lambda calculus and set-theory, can be defined?