Thirukkural 845

கல்லாத மேற்கொண் டொழுகல் கசடற வல்லதூஉம் ஐயம் தரும்.

kallātha mēṟkōṇ ṭozhugal kasaḍṟa vallathūum aiyam tarum.

To undertake what you have not learned is to travel blind. Even a practiced hand may stumble in the dark.