In plain words
It must be there, but we must travel keeping it under control. In just the same way we must have a mind. We must try to use it in the right form. It is because we use our Manas today as it pleases that we fall into so many kinds of sorrow, worry and hardship. That is exactly where we need Buddhi most. Buddhi develops the power of man. Humanness is the working of Buddhi itself. The working of that Buddhi takes the whole world into itself. This is the secret of Manas.
We have gone round the whole world. We have toured all of Hyderabad, toured all of Madras, toured all of Bombay. We have seen many mountains, seen rivers. We have seen so much more. How many sights we have seen! How many people we have seen! How many things we have seen! All of them, in a very subtle form, have been printed in our mind. How have you packed the sight of so vast a world into so small a mind? Is there room anywhere for so big a mountain to be in your mind? So we can see how vast Manas is. The whole world is held within us. That is why mano mūlam idaṃ jagat. The whole world is the working of Manas. In that world I too am one. In this world, made so much of Manas, how much is this body? Only when we enquire well into such vastness, and into something of the atom's smallness, does Vairāgya come to us well. In so vast a world, how much is this body! When we see it as a mere atom, what a humiliation it seems to us! Hence the Vedānta: aṇoraṇīyān mahato mahīyān; in the great, the very greatest. In the atoms the very subtlest; the principle in these two is one. A banyan tree grows very big. You may have seen it in Madras. In the Theosophical Society, a big tree. But the seed in it is very small. In this small seed itself the great big tree is held. In this big tree the small seed is held. In just the same way you may think our Manas very small. Right within it is infinity. In this infinite world the principle of Manas, slight and small, an atom in form, is held too. That is I myself; I am that. So long as there is attachment to the body, this Ānanda, this oneness, will not make sense to us. So long as the sight is gross, this infinity too will not make sense to us. A small example.
There is a big cement trough. Or we go into a tank and look. When we look into it, our reflection appears in that water. What do you say? 'That is I myself,' you say. But when you enquire, that is not I. That is I myself; I am not that.