In plain words
It is Manas itself that goes on taking all these forms. To the one Manas alone all these names keep coming. When one and the same brāhmaṇa is cooking, we say, "the cooking brāhmaṇa". When the same brāhmaṇa is doing pūjā, we say, "the priest brāhmaṇa". When the same brāhmaṇa is teaching, we say, "the teacher brāhmaṇa". When the same brāhmaṇa is reading out the almanac, we say, "the almanac brāhmaṇa". The brāhmaṇa is one alone. Even so, his names keep coming differently according to his work. So too, when Manas takes on many activities, the names come: this same one is Buddhi, this same one citta, this same one Ahaṅkāra, this same one Manas. All these are other names for Manas. There is nothing separate called the antaḥkaraṇa. All our instruments can be seen on the outside. The eye can be seen on the outside. The ear hears what is outside. The nose breathes in the outside air. All these are outer instruments. Manas, Buddhi, citta and Ahaṅkāra are the inner instruments. They are the tools that work inside. So they got the name of inner instruments. Even when we enjoy through the outer instruments, with the sacrifice of the antaḥkaraṇa, this becomes enjoyment joined with sacrifice. So first of all we must purify the antaḥkaraṇa.
Every human being must tie on the wristband of a vow for the doing of karma. Without karma, the citta is not purified. So the Veda said, Cittasya śuddhaye karmaḥ. For the purity of the citta, every human being must perform good karmas. What are these good karmas? To perform karmas with no wish for the fruit, with no selfishness at all, is called niṣkāma karma. This very niṣkāma karma they named sacrifice joined with enjoyment. The Īśāvāsya Upaniṣad went on teaching this kind of sacrifice joined with enjoyment in many ways. One must never be a mere man without action; the body came forth for the sake of karma. Karma is what makes time sacred. Time is something joined with duties of its own kind. A life of time, karma, cause and duty is man's chief goal. On what path are we setting the humanness that is joined with time, karma, cause and duty? Always selfishness, self-interest; selfishness, self-interest alone. How then will enjoyment joined with sacrifice be gained?