The honey-doctrine of the Bṛhadāraṇyaka. All things are mutually the honey of all other things — and the immortal self within each is the same self. A cosmological statement of non-duality expressed through the sweetness of mutual dependence.
The Madhu-brāhmaṇa (honey-doctrine) is one of the most lyrical passages in any Upanishad. Its logic is simple: every element of the world is the honey (essence, nourishment) of every other element, and the immortal self within each element is the same self.
The earth is the honey of all beings — it nourishes everything that lives on it. All beings are the honey of the earth — they are what the earth produces as its flowering. The waters are the honey of the earth, and the earth is the honey of the waters. Fire and speech. The sun and sight. The wind and breath. The directions and hearing. The moon and mind. Lightning and energy. Thunder and sound. Space and the self. Each pair held in mutual sustenance — each the honey of the other.
And within each element there is an immortal self. The immortal self within the earth is the same immortal self within all beings. The self within the waters is the self within the living being who depends on water. The self within fire is the self within the speech that fire symbolises. Layer by layer, the teaching arrives at its point: this immortal, Brahman-self within each element is the same self. There is no plurality of selves — there is one self appearing as the vitality within every element of existence.
The Madhu-brāhmaṇa's structure is a systematic application of the antaryāmin (inner controller) concept from Bṛhadāraṇyaka 3.7: Brahman is the consciousness within each element that makes it what it is. The honey teaching extends this: not only is Brahman the inner controller of each element, but each element is the essence (honey) of the others — they are mutually constitutive. This mutual constitution is the empirical face of non-duality: from the ordinary standpoint, things are separate and distinct; at the deeper level, each thing is what it is only in relation to everything else, and what underlies all those relations is one consciousness.
The phrase repeated throughout — ayam ātmā sarvasyāpyayaḥ (this self is the refuge of all) — is the Madhu-brāhmaṇa's positive conclusion: the self is not just the ground of one element but the refuge of all. Apyaya means both refuge and return-point: everything dissolves back into this self, and everything arises from it. The honey image captures the bidirectionality: the bees collect from flowers (all beings depending on the self), and the honey nourishes the bees in return (the self being what sustains all beings).
Śaṅkara's Bhāṣya on the Madhu-brāhmaṇa focuses on the repeated phrase sa yo h'ayam asmin brahmapucche prati tiṣṭhati — "he who rests in this self, which is the tail (foundation) of Brahman." The image of Brahman's "tail" is unusual. Śaṅkara reads it as an image of support: just as a tail supports an animal's balance, this self (which the teaching has been progressively uncovering) is the final support of Brahman itself — meaning: Brahman as the ground of the world is the same Brahman that is the self of the teaching's recipient. The cosmological and the soteriological converge: the Brahman that underlies the universe is not a different Brahman from the one that is your own innermost self.