Let us observe two things \(A\) and \(B\). Now suppose these are the only things to observe. We would say that there doesn’t exist more than \(A\) and \(B\), but how could we describe existence?
\(A\) is simply \(A\) and \(B\) is simply \(B\)? We observe both things in relation to each other, to differentiate and then individualize. \(A\) cannot simply be \(A\) for itself. \(A\) is here because it is not \(B\) and \(B\) is there because it is not \(A\). \(A\) and \(B\) exist via their relation to each other. \(A\) for itself would have to mean an absolute thing, which is equivalent to no thing. Without demarcation the needed contrast to observe any thing would be missing.
For \(A\) to exist, we already needed \(B\) and vice versa. \(A\) and \(B\) are not possibly in relation, but inevitably. We cannot isolate an identity to designate an individual, \(A\) or \(B\). We can conclude that everything what exists is defined by what it exclusively is not. Moreover the chain of existence cannot start with a single absolute thing, which wouldn’t be differentiable from no thing. It has to start with a multitude of things in relation.
Duality of existence holds an explanation for Wave-particle duality on a philosophical level. The wave might be a witness for the existence of the particle and vice versa.
The identity of any thing depends on all other things. Therefore I do not know who I am, but suspect who I am not.