As a noun, the word reason has two essential meanings. The first is the more colloquial; a reason is the grounds on which something is done or believed–it is the purpose that explains an action or the premise that justifies a belief. But in the second sense, reason itself is not the purpose for an action but the very mode of acting in which purpose is considered, nor is it the premise of a belief but the very mode of believing in which premises are necessary. In other words, to be rational is to have a reason for those actions which you perform and those thoughts which you believe.
In highly impractical epistemological contexts, the notion often becomes relevant that one may just as well employ some particular mode of thinking as any other. There is no reason, in the first sense of the word, for reason itself, in the second sense of the word. And once an alternative mode of thinking is employed, the intellectual performing this thought experiment will very likely find himself just as convinced of an alternative metaphysical paradigm as he was of his original view. Thereupon, he will inevitably reach the terrifying conclusion that all his thoughts, opinions, and beliefs are entirely and inescapably arbitrary; for because he cannot justify any one mode of thinking over another, he may never categorically support any beliefs that arise in a particular mode–all may be falsified by a mere shift in perspective.
However, as we examine the thought experiment from a safe distance, it is clear that it has a number of detrimental failures. For one, the whole basis of the experiment is rational. By this I mean first that it is performed for a purpose, that of exploring the validity of a rational paradigm. And then once it is performed, the results are interpreted from within that same paradigm–what dissatisfies the experimenter is the fact that he cannot have a reason for what he believes. But his being disappointed by a want of reason proves that he is still, by definition, functioning rationally, and in fact has never left that mode of thinking. He began and ended with purpose, so that even when he supposed himself to be thinking “irrationally,” he was still behaving in his particular manner for a reason, the purpose of experimentation. The human mind is incapable of performing an action without purpose, and therefore, can never truly escape the rational scope.
What our experimenter underwent, then, was a dissociative experience. He found himself, for a moment, in a scope of thought that denies its premise. For a more thorough discussion of this phenomenon, see Is Hypnosis self-evident? A Concise Philosophical Inquiry.
Why do you believe in reason?
The question assumes that I do.