It is that one who hears himself when speaking, and understands it duly. Once, again, is that heard from him when he speaks. There is no second preceptor.
शृणोत्ययं प्रोच्यमानं गृह्णाति च यथातथम्। पृच्छतस्तदतो भूयो गुरुरन्यो न विद्यते॥