He was saying more than one thing when he said this (it’s his brand of humor mixed with wit)… but mostly that the only sure thing is beyond fully grasping—it surpasses understanding for the same reason you are the only one who will experience you-ness—and that kind of knowledge isn’t taught by reading a book or word of mouth or a science experiment—it isn’t taught. It just is.
Isness is ontology, not epistemology.
He was basically pulling a Descartes before it was cool to pretend you could actually argue against Descartes.
There, a bread crumb.