Thirukkural-On Virtue-Truthfulness-Kural-293

Do not utter falsehood about what your conscience knows to be true; For later, your own conscience will continue to hurt you.

Uttering a lie about matters which one knows in one's heart to be true, would lead to one's own conscience becoming an accusing witness. The idea of one's own heart or the conscience, becoming the accusing witness, "Nenju kariyaadhal" has been quoted by Parimelalagar from "Kalithogai" (Kalithogai 125, 1) to endorse his interpretation.

Another line, which has a similar meaning is the following from 'Kurunthogai'.

"Arikari poithal aannroarkillai"

'Every soul', says Plato, 'parts unwillingly with Truth', therefore, it speaks with a thousand tongues, as Shakespeare would say, condemning one for a villain.

In fact modern scientific criminal interrogation by the Police is based on the psychological principle that there is an inner urge, arising from the pricking of his own conscience, in every man, who has committed a crime or uttered a lie, to come out with the Truth and the purpose of interrogation, according to Hans Gross, should be to create the circumstances which will facilitate the process.