Regarding the previous lesson (2) in Introduction to Philosophy

Hello Rabbi, I had a specific question about the previous lesson and I didn't get to ask in real time.
The Rabbi singled out Descartes' cogito as a symbol of rationalist, rather than empiricist, philosophical proof.
I wanted to clarify the point, I didn't fully understand why it wasn't an observation post, when I walk it's definitely an observation post .
But even when I think, it is an observation. I observe that I think , and from this I conclude that I exist. It is simply a necessary observation that cannot be deceptive, unlike other observations.
I would be happy if the Rabbi could clarify this point for me, thanks in advance.