In plain words
Buddhi is close to the Ātma. Because it is close to the Ātma, the influence of the Ātma, the radiance of the Ātma and the good fruitfulness of the Ātma have the chance to set this Buddhi right. So nearly 90 per cent of all the power of the Ātma can come upon the Buddhi. It is only after this Buddhi that the body spreads over the Manas. The influence of this Manas spreads over the senses. The influence of the senses spreads over the body. We have thought of Buddhi as a clean mirror. It is very hard for the rays of the sun to enter our house. If you want the sun's rays to enter a house that lies in deep darkness, turn a mirror towards the sun and then turn it a little towards the house: the sun's reflection falls on the mirror, and the mirror's reflection enters the house. These rays of the sun cannot fall into the house by themselves. They have to enter the house through the mirror alone. So the rays of the sun that fall on the mirror lessen their own power by degrees and then show that reflection in the house. Here is another small example for this. The place where a spring of water rises is pure. But as it flows on by degrees and comes to the level where people use it, it picks up impurity. In just the same way the influence of the Ātma falls on the sun, that is, falls on the Buddhi, falls on the Manas, from the Manas falls on the senses, and by the time it reaches the body through the senses it has picked up impurity in full. So when we shape this sacred Buddhi to be pure, the influence of this Buddhi can fall directly on the body. So man must strive for the purity of the Buddhi and the spotlessness of the Buddhi.
Among the ten Upaniṣads the Taittirīya Upaniṣad is the chief. This Upaniṣad has described the Buddhi as a bird. It has declared its head to be śraddhā. It has named the right-hand pakṣa, that is, the wing, Ṛta. It has declared the name of the wing on the left to be Satya. It has given the body in the middle the name Mahat-tattva. It has named its tail yoga. That is, for the bird called Buddhi, śraddhā is the head and the wing on the right is Ṛta.