"Let's Get Metaphysical, Metaphysical ..."

That post title is a reference to the Olivia Newton John song "Let's Get Physical." You probably knew that, cuz you're about as old as I am. :-)

I am currently fascinated by the concept of how our base of knowledge is built upon key assumptions and assumed "facts." If we peel back the onion all the way, what is the primary, singular, fundamental "first assumption?"

Is it this: I assume I really exist?