What has been clear to me for quite a long time, for years, is that equal and opposite realtivities are possible because they sum to zero.  The math was simple: +1 -1 = 0.

And I would wonder:

Why is there, why does there have to be a +1 / -1 ?

What occured to me today is this:

for nothing to exist there has to be +1 / -1.