Thirukkural- On Virtue- Hypocrisy-Kural-279

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The arrow that is smooth and straight is cruel,While the crooked 'yal' gives sweet music;

Therefore our judgement on things should not be based on appearances but on performance.

Men and things are better judged by their actions and not by their appearances; for the streamlined arrow is deadly while the music of the crooked yal is sweet.

As the Bible says, we should know them by their fruits.

'Ye shall know them by their fruits,
Do men gather grapes from thorns or figs from thistles'?

(Matt 7,16)