In plain words
That is not all. M means Mind, B means Body, A means Atma. Mind, body, Ātma: what makes these three known is this very MBA. The mind resolves, and the body puts it into practice. The Ātma watches whether these are carrying out their duties or not. The Ātma is eternal. The mind is unsteady. You must set it on the right path. “I am a human being. Is my mind such as befits a human being?”: you must enquire in this way. And then, what must the body do? The moment the mind has decided, the body must not go into action. Using Buddhi, you must enquire, “Is what the mind has decided right or wrong?” If what the mind has decided is good, you must put it into practice. Every human being must recognise the principles of body, mind, Buddhi and Ātma. First of all you must purify the mind. The mind is master of the senses. But what is the use? It is not controlling the senses. Only he who controls the senses is the real ‘master mind’. So, Master the mind, be a master-mind.
And what is the Ātma principle like? It stands as the onlooker of everything; it watches the work that mind and body do, but does not itself interfere in it. Is the mind doing its own work? Is the body carrying out its own actions? So it goes on watching both, as their onlooker.
Vālmīki's questions, Nārada's answer
At one time, Vālmīki to Nārada: “Swami! Is there in this world one who keeps to the word of truth? Is there one who enjoys Ānanda at all times? Is there one whose face sheds smiles at all times? Is there one who does no wrong? Is there one who forgives those who have done wrong? Is there one who shows gratitude for any help received, however small? Is there one who leads his life by Satya and Dharma?...” In this way he asked eleven questions. Then Nārada: “Vālmīki! Not eleven: Śrī Rāma, the lord of eleven thousand virtues, is here in this very world, having taken human form.