Brahma Sutras – According to Shankara 3-4 4-5
Topic 4 - The stories recorded in the Upanishads do not serve the purpose of Pāriplavas and so do not form part of the ritualistic acts. They are meant to glorify the Vidyā taught in them
Sutra 3,4.23
पारिप्लवार्था इति चेत्, न, विशेषितत्वात् ॥ २३ ॥
pāriplavārthā iti cet, na, viśeṣitatvāt || 23 ||
pāriplavārthāḥ—For the purpose of Pāriplavas; iti cet—if it be said; na—not so; viśeṣitatvāt—on account of (certain stories alone) being specified.
23. If it be said (that the stories that occur in the Upanishads are) for the purpose of Pāriplavas, (we say) not so, because (certain stories alone) are specified (by the Śruti for this purpose).
In the Aśvamedha sacrifice, which lasts for one year, the sacrificer and his family are expected to hear at intervals the recital of certain stories. These are known as Pāriplavas, and form part of the ritualistic acts.
The question is whether Upaniṣadic stories also serve this purpose, in which case they become part of the rites, and this means that the whole of Jñāna kānḍa becomes subservient to Karma-kāṇda.
The stories referred to are those relating to Yājñyavalkya and Maitreyī, Pratardana and so on, which we find in the Brihadāraṇyaka, Kaushitaki and other Upanishads.
This Sutra denies that they serve the purpose of Pāriplavas, for the scripture specifies the stories that are meant for this purpose. Any and every story cannot serve this purpose. Upaniṣadic stories are not mentioned in this category.
Sutra 3,4.24
तथा चैकवाक्यतोपबन्धात् ॥ २४ ॥
tathā caikavākyatopabandhāt || 24 ||
tathā—So; ca—and; ekavākyataḥ-pabandhāt—being connected as one whole.
24. And so (they are meant to illustrate the nearest Vidyās), being connected as one whole.
The stories not serving the purpose of Pāriplavas they are intended to introduce the Vidyās. The story form is meant to catch the imagination of the student, who will thereby be more attentive to the Vidyā described.
Topic 5 - Sannyāsins need not observe ritualistic acts, as Knowledge serves their purpose
Sutra 3,4.25
अत एव चाग्नीन्धनाद्यनपेक्षा ॥ २५ ॥
ata eva cāgnīndhanādyanapekṣā || 25 ||
ata eva—Therefore; ca—and; agni-indhanādi-anapekṣā—no necessity of lighting fires etc.
25. And, therefore, there is no necessity of lighting fires, and so on.
In Sutra 3.4.1 it was said that the knowledge of the Self results in the attainment of the highest Puruṣārtha or goal of life.
Therefore the lighting of fires for sacrifices and other similar acts enjoined on the householders etc. need not be observed by Sannyāsins, as Knowledge alone fulfils their object.