fromkentucky:
If there are only two states of being, existence and non-existence, then anything that qualifies as NOT(non-existing) must exist, making existence the default assumption.
It isn't clear what you mean by "default assumption" .
Asumption is defined in Black's Law as "A fact or statement taken for granted; a supposition < a logical assumption>"
Supposition is defined as "An assumption that something is true , without proof of its veracity [validity or truth]; the act of supposing."
Taking these definitions into consideration, when one posits (states as a principal or a fact) a particular postulate (a position or supposition assumed without proof) it establishes nothing!
I wonder if you are trying to convey the idea of 'default position'. Either way the outcome is the same.
For instance, those who posit the existence of a particular god, and those who posit the non-existence of god stand in an equal position. Both persons are positing concepts that are assumed (supposed without proof) by them. Their assumed position (based upon the act of positing) is not established firmly by their supposition, and cannot logically be, if we hold ourselves to the principles of intellectual honesty and logical consistency.
fromkentucky:
After all, if the default assumption is non-existence [existence], wouldn't you be spending the entire time trying to prove a negative (non-existence)?
Shouldn't the default assumption be that a proposed claim is invalid until proven otherwise?
You later corrected your statement as set forth above in brackets. Either way, any default assumption whatever its nature should be considered invalid until proven otherwise. What is true for the hypothesis of biblical creation likewise applies to the big bang hypothesis. Neither of these postulates have ever been firmly established. Both are nothing more than hypothesis.
As to solving the question of 'whether reality is composed of a state of existence or non-existence?', humans must first agree on a starting point. If the starting point is one of observation, (which is all we really have with which to weigh) then the most logical conclusion is that the state of existence is real (reality).
The most we as humans can presently know with the tools of observation that we are equipped with is 'what is the most logical position'. That position can be discovered, and should be recognized. Having made the individual discovery of "what is the most logical position', the incumbant duty is to move forward in our search for additional enlightenment from there.