You never know what you don't know. That might take some explaining. If you knew everything in the world about, say, painting, and that's the only form of art you ever knew existed, you would think you were pretty knowledgeable about art. You might even call yourself an expert on all art the world has ever produced, until someone showed you a sculpture. Until then, you wouldn't know that you didn't know everything about art. You need to have a little knowledge to know that you don't have it all. You also need a little wisdom and humility.
Mokalus of Borg
PS - You can admit you don't know everything without knowing what you're missing.
PPS - But the more you know, the more you realise how little you know.