KeU 2.3
यस्यामतं तस्य मतं मतं यस्य न वेद सः । अविज्ञातं विजानतां विज्ञातमविजानताम्
He who thinks he knows It not, knows It. He who thinks he knows It, knows It not. The true knowers think they can never know It (because of Its infinitude), while the ignorant think they know It.
Translated by Swami Paramananda
'He by whom it (Brahman) is not thought, by him it is thought; he by whom it is thought, knows it not. It is not understood by those who understand it, it is understood by those who do not understand it.
Translated by F. Max Müller