That which purifies the impure mind is tapas. That which regenerates the lower animal nature and generates divine nature is tapas. That which cleanses the mind and destroys lust, anger, greed etc., is tapas. That which destroys tamas (dullness) and rajas (impurity) and increases satva (purity) is tapas.
That which steadies the mind and fixes it on the eternal is tapas. That which arrests the outgoing tendencies, destroys vasana (habits), egoism, raga dvesa (likes and dislikes) and generates dispassion, discrimination and meditation is tapas.
This is the third limb of niyama (discipline) in raja yoga. It is one of the three items of kriya (dynamic) yoga. Tapas means austerity or practice of penance. The man of tapas is brilliant like a blazing fire.
Tapas means restraint of the senses and meditation. Also tapas leads to control of mind. Standing on one leg, raising one hand up for a long time is also tapas, but this is tamasic (dull) tapas of an ignorant man.
Pancagni tapas is sitting in the midst of four fires in the hot sun, having the hot sun as the fifth fire. Vairagis practise this very often. Desire moves the senses; desire can be controlled only if the senses are curbed. Tapas annihilates desires. Annihilate tapas abhimana (pride of austerity). The unintelligent tapasvin (ascetic) is always irritable, hot-tempered and proud. Practise intelligent tapas.
Mental tapas is more powerful than physical tapas. He who bears heat and cold does physical tapas. He increases his power of endurance, but he may not be able to bear insult. He will be easily upset by a harsh or unkind word. He may take revenge and do tit for tat. He has no control over the mind.
He has disciplined only the physical body. To keep a balanced mind in all conditions of life, to bear insult, injury and persecutions, to be ever serene, contented and peaceful, to be cheerful in adverse conditions, to have fortitude in meeting danger, to have presence of mind and forbearance, are all forms of mental tapas.
"The men who perform severe austerities, unenjoined by the scriptures, wedded to vanity and egoism, impelled by the force of their desire and passion, unintelligent, tormenting the aggregated elements forming the body and Me also, seated in the inner body, know these to be demoniacal in their resolves". (Gita. Chapter XVII).