Of course it is. We all have a few axioms in our lives that we can't prove or substantiate. For some people that can be belief in the existance of God, from which a whole comforting system of objective morality can spring. Frankly, I envy them.
For me, the inescapable truth of my reality is that I live in a world that exists against staggering odds for no greater purpose, and that I have been blessed with this gift of consciousness and reason not by a benevolent god, but the caprice of quantum improbability. This is a horrible universe, and one that I try to think about as little as possible, but the fact is I am completely unable to bring myself to believe in a world of absolute morals, benovolent creators and merciful saviors. It's a big enough leap to consent to the shared illusion of mutual reality - when it comes down to it, I have very little assurance that anything outside my own head exists.
So yes, it's an article of faith, but we all have to start somewhere. Even the big bang theory can't tell you what happened at minus 5.4e-44 seconds before time began. In this world, How is approachable by human knowlege, but the Why will always lord over us as a cruel and ineffable mistress.
For the record, I have two axioms I live by:
1.) The world keeps going when I close my eyes - that is, other people exist on as permenant a basis as I do, and
2.) Love is real.
I have no reason for believing either of these, but I couldn't bear to live in a world where they were false. And since oblivion is boring, and basically promised to me anyway in less than 80 years, I choose to suspend my disbelief.