By His Divine Grace A. C. Bhaktivedānta Swami Prabhupāda
Tapah is explained in the smrti-śāstra as follows: manasaś cendriyānām ca aikāgryam paramam tapah. “Complete control of the mind and senses and their complete concentration on one kind of activity is called tapah.” Our Krishna consciousness movement is teaching people how to concentrate the mind on devotional service. This is first-class tapah. Brahmacarya, the life of celibacy, has eight aspects: one should not think of women, speak about sex life, dally with women, look lustfully at women, talk intimately with women or decide to engage in sexual intercourse, nor should one endeavor for sex life or engage in sex life. One should not even think of women or look at them, to say nothing of talking with them. This is called first-class brahmacarya. If a brahmacārī or sannyāsī talks with a woman in a secluded place, naturally there will be a possibility of sex life without anyone’s knowledge. Therefore a complete brahmacārī practices just the opposite. If one is a perfect brahmacārī, he can very easily control the mind and senses, give charity, speak truthfully and so forth. To begin, however, one must control the tongue and the process of eating.
In the bhakti-mārga, the path of devotional service, one must strictly follow the regulative principles by first controlling the tongue (sevonmukhe hi jihvādau svayam eva sphuraty adah [Brs. 1.2.234]). The tongue (jihvā) can be controlled if one chants the Hare Krishna mahā-mantra, does not speak of any subjects other than those concerning Krishna and does not taste anything not offered to Krishna. If one can control the tongue in this way, brahmacarya and other purifying processes will automatically follow. The path of devotional service is completely perfect and is therefore superior to the path of fruitive activities and the path of knowledge. Quoting from the Vedas, Śrīla Vīrarāghava Ācārya explains that austerity involves observing fasts as fully as possible (tapasānāśakena). Śrīla Rupa Gosvāmī has also advised that atyāhāra, too much eating, is an impediment to advancement in spiritual life. Also, in Bhagavad-gītā (6.17) Krishna says:
“He who is temperate in his habits of eating, sleeping, working and recreation can mitigate all material pains by practicing the yoga system.”
[Bhaktivedanta Purport to the Srimad Bhagavatam 6.1.13-14]