Thirukkural 849

காணாதான் காட்டுவான் தான்காணான் காணாதான் கண்டானாம் தான்கண்ட வாறு.

kaaNaathaan kaaTTuvaan thaan kaaNaan kaaNaathaan kaNdaanaams thaan kandda vaaru.

The blind man shows the way to one who sees, though he himself has not seen. Yet he who has seen walks as he has been shown—and so becomes blind to his own judgment.