To be completely strict and honest -- even though those are, too, just learned mind constructs pointing at (and denoting) what is, beyond "I am" I cannot say anything more about the nature of reality without drifting into speculation and/or ambiguity. Thus, "I am" is my only certainty. I cannot define cause, neither essence, nor purpose of anything-- I am what I am.

When I realized it several years ago, I discovered that it's quite an ancient knowledge. But limiting myself to only those two words would be rather inconvenient, and also, I'm a rather talkative fellow. From time to time at least.

Hence, "I am thinking", although a fact (more or less :D), is for me but a manifestation, a particular form. How it took shape? Beats me. Then again, in those ancient sources I also discovered that that process (of "arising") is allegedly inscrutable. But when it comes to cosmology, I'm pretty optimistic and open to new information. Still, it's not my primary concern and I'm more interested in the practical day-to-day aspects of realization.