IIRC, his reasoning was that he could not take knowledge and truth for granted, as that would mean assuming that his senses were accurate. He then proceeded to reason that he had to start from scratch, and only build knowledge based on truths he could land on without sensory inputs.

And that's how he landed on Cogito Ergo Sum.

Not really a ripoff, but without his famous conclusion, it could seem that all he's about is his starting point, which is very similar to that of Socrates.