Well, just the statement itself suggests that it's Da Vinci's "perception" that that's where our knowledge originates. Either way, it's a debatable theory. For a thorough investigation may I suggest you look into epistemology. It's the branch of philosophy dealing with the nature of knowledge in all it's applications. Have fun!
It means that there is no such thing as pure or absolute knowledge which is just out there waiting to be discovered. We can only know or understand anything as far as we can perceive it/see it.