In plain words
Those without Vidyā and tapas cannot become pure
“Adhyātmavidyā vidyānām”: this is stated in the Gītā too, in the ‘Vibhūti Yoga’. All the branches of learning are rivers. Adhyātmavidyā is the ocean. Just as all rivers join the ocean, all branches of learning merge in Adhyātmavidyā. More than that: just as, where they meet the sea, all the rivers lose their separate names and forms and take the form of the undivided ocean, so in their union with the ocean of Brahmavidyā all outer learning gives up its form, name and the rest. “Vidyā tapobhyāṃ pūtātmā.” Through Vidyā and through tapas man becomes pure. Vidyā is of two kinds, outer learning and Brahmavidyā. Outer learning came into being only for man's livelihood. To study high subjects, gain degrees, win great posts and make life comfortable, free of worry and care. Every post, from scavenger to prime minister, belongs to outer learning. That is why all outer learning has been called study for the belly's food. Brahmavidyā, on the other hand, has the power and the capacity to bring all men to fulfilment. It opens and creates ways both to the concerns of this world here and to the happiness of the other world there. So Brahmavidyā has been reckoned higher than all the learning on earth. The divine power that can carry everyone across belongs to Brahmavidyā alone, not to outer learning. Brahmavidyā makes Parabrahman known. Tapas unites one with Parabrahman. Vidyā is knowledge; tapas is what is to be known. The first is indirect, the second direct. Guru means a great one. That is, he must be skilled and wise in both of these. Just as both soap and clean water are needed to remove the dirt stuck to the cloth one wears, so both Vidyā and tapas are needed to remove the dirt stuck to the Manas. Only then can the antaḥkaraṇa be purified. Just as a cart without two wheels and a bird without two wings cannot travel, so those without Vidyā and tapas cannot become pure. Tapas does not mean hanging like a bat, feet up and head down. Nor does it mean leaving property and possessions, leaving wife and children, holding one's nose and starving one's belly. To make the three, body, speech and mind, one, and to act with the threefold purity, is called tapas. This must be done together with one's duty. The striving for the Ātma's contentment is itself tapas. In the Gītā, the guru and the disciple are ideal figures. The disciple is the embodiment of fitness, the guru the embodiment of the Avatāra. The disciple is the best of men, the guru the Supreme Person. Pārtha, the great soul, is the disciple; Kṛṣṇa, the Paramātma, is the guru. The bearer of the bow is the disciple; the lord of yogis is the guru. Only when guru and disciple are of this kind is Brahmavidyā attained.