The above paradox is quoted by G.K. Chesterton in the very first of the Father Brown detective stories and attributed to (Edgar Allen) Poe. There was another American who talked about known knowns, known unknowns, unknown knowns and unknown unknowns. I actually think his formulation was quite neat, unlike his legacy.
Poe was right.
Occasionally in this blog I depart from questions of political philosophy and suchlike to talk about something serious - birdwatching. A few days ago I went on a longish walk along our local river, the Stour, which for much of its course forms the Essex/Suffolk border. I was in the attractive area known as Dedham Vale, crossing damp meadows by a path close to the river, and was due a stop for coffee and sandwich. I found a suitable just-about-fallen willow and sat down. There were large black-and-white (Frisian?) cows nearby. As I ate and drank, one cow approached close. I guarded my food. I did not think there was any point in guarding the other things I'd put down. Putting the sandwich box away, I looked up to see the cow making off with my binoculars dangling from its mouth by the strap. As the river was close, a disaster was possible.
Fortunately my yell caused the cow to drop the binoculars, and on grass, not in the water.
Never underestimate cows.