Doubt

Doubt exists. I cannot doubt that fact.
But in its very certainty there lies
A paradox, to which I must react
With doubt - which means the paradox then dies.
But in its death, there should be certainty
And that reanimates it once again;
How can a thing by being cease to be
And what am I to do about it then?
It's fortunate that doubt is not alone
And therefore I can doubt some other point
For should I live in such a monotone
I would by Schrodinger be out of joint
For as observer, I would always see
The half-chance of my ceasing to be me.