Hi, Austin.
What would it be that IS?
Existence?
A Maker?
What is it that IS?
(No definition; no substance to IS.)
(Assumption false, for I do not know.)
It IS pretty simple:
is : form of be
be (EXIST) : verb [I] : to exist or live
You are correct: it IS what the individual chooses to know, or not know, in order to be the creator of his/her own individual experience. You can't deny that our point of view about all that we encounter is not unique and just IS what it IS.
Until science can read/record what we're actually thinking from the brain's electrical impulses, it remains unquantifiable.