What Descartes left out was this : think what exactly ? Was he being stylish, mysterious or just careless ? We'll never know - and thank goodness. Philosophy would have lost some serious shine had that line been,say, "I think of shopping therefore I am in the mall."
I wish I'd known about the philosopher Immanuel Kant's Critique of Pure Reason when I found out that critique and criticism weren't the same. It would have both lessened the shock and illustrated the difference. For Kant's critique is apparently all praise for pure reason.
I think pure reason has its limits though. It's the lifeblood of civilization and all that of course but not particularly effective in such situations as when confronting a raging large man who has it in for you or a raging little ultra-nationalist who has it in for your views.
I'm inclined most towards the philosopher David Hume. Hume is known for empiricism and skepticism. I have no idea what empiricism is - nothing to do,I'm told, with umpiring or even empire -but I'm skeptical enough to never go anywhere near philosophy. Which is why I should stop here.