I read "Notes From Underground" a few weeks ago, so I don't clearly remember the context of this passage, but I did love the book dearly and I've just finished reading Descartes, so I'll try and give you my thoughts on the matter.
That is indeed almost word for word Descartes. Indeed, I think the similarity is so striking that it must have been very much intentional, and I suspect that Dostoyevsky included it as yet another ingredient thrown into the potpourri of the Underground Man's expansive and self-contradictory ideas about life and knowledge about philosophy and other such subjects. Dostoyevsky himself was noted for saying "Philosophy is dead," noting that more than logic is needed to explain life, and his work seems to demonstrate this belief. However, I think that, based on his other work, Dostoyevsky does find some truth in the statement. "Crime and Punishment" seems to be all about this in my opinion--the "thinkers" get things so muddled up that they forget what's good for them and wind up killing old ladies. Anyway, both "Notes From Underground" and "Crime and Punishment" are masterpieces and among my favourite books.
A posture after my own heart

welcome broK