ब्रह्मणो हि प्रतिष्ठाऽहममृतस्याव्ययस्य च।शाश्वतस्य च धर्मस्य सुखस्यैकान्तिकस्य च॥
brahmaṇo hi pratiṣṭhāham amṛtasyāvyayasya caśāśvatasya ca dharmasya sukhasyaikāntikasya ca
Word by word
The verse says
For I am the ground of Brahman — of what does not die, of what does not wear out, of the lasting way, and of a happiness that has no end.
In plain words
The last line of the chapter, and it is a foundation claim.
Pratiṣṭhā — the base something stands on.
Not that he is Brahman. That Brahman rests on him, along with three other things the book has spent fourteen chapters recommending.
Where you've felt this · in growing older
Your grandmother's recipes are written on cards in her own hand and the family treats them as the authority. But she did not learn to cook from cards. Somebody stood next to her for ten years, and the cards are an attempt to write that down afterwards.
Going deeper
Putting the personal under the impersonal, rather than the other way round, is the claim this chapter ends on and it is contested.
The usual philosophical instinct in this tradition runs the other way: Brahman is the ultimate reality and the personal form is how it appears to people who need a face. This verse reverses the order. Brahman rests on him, and so do immortality, the lasting way and unending happiness.
The schools divide here sharply, and which way you read it determines a great deal downstream.
What can be said without taking a side is what it does to the chapter. Fourteen chapters of analysis — fields, strands, elements, categories — are placed on a foundation that is described in personal terms. Whatever else that means, it means the book does not regard the analysis as the deepest thing it has to offer, and it has now said so three times in four chapters.