BG 17.16-18
Mental happiness, equilibrium, silence, self-control, purity of nature—these are called the austerity of the mind. This threefold austerity, performed by men with the utmost faith, without desire for fruit, harmonised, is said to be pure. The austerity which is practised with the object of gaining respect, honour and worship, and for ostentation, is said to be of passion, unstable and fleeting.