In other words, "we know what we don't know".
I gave a true historical analogy of how that could not be the case, I gave a hypothetical example of how that could not be the case, and I gave a quote of someone thinking physics was basically solved right before a massive change in our basic understanding of physics.
It's easy to imagine that there is something analogous to radio waves, that are all around us, but we are unable to detect them with current technology (dark matter might be one possibility).