William James wrote somewhere (I'm feeling too lazy to look it up) that there is a class of cases where the word "it" has no clear antecedent, yet where this apparent ambiguity is perfectly acceptable as a matter of idiomatic English. "It is raining," after all, is a simple statement of fact. Nobody, hearing it, scratches his head and askes, " what is raining?" The phrase in question conveys the same meaning as "Rain is falling at this moment," yet conveys it in half as many syllables. The lesson: we can't deduce a metaphysical fact from a grammatical subject. So what of the pronoun "I" in the sentence "I think"? This is one crucial problem with Descartes' famous reasoning. The thoughts with which he tormented himself in the course of his methodical doubt, the thought that there might be a powerful evil demon, etc., and the thought "I think" itself, might all be rain, as it were, coming ...