(मूर्तामूर्तब्राह्मणम्) द्वे वाव ब्रह्मणो रूपे ।मूर्तं चैवामूर्तं च ।मर्त्यं चामृतं च ।स्थितं च यच्च ।सच्च त्यं च ॥बृह. २,३.१ ॥
In plain words
There are two forms of Brahman: the formed and the formless, the mortal and the immortal, the fixed and the moving, the being and the beyond.
The commentators
Bṛhadāraṇyakopaniṣadbhāṣyam
तत्र प्राणा वै सत्यमित्युक्तम् । याः प्राणानामुपनिषदः, ताः ब्रह्मोपनिषत्प्रसङ्गेन व्याख्याताः- एते ते प्राणा इति च । ते किमात्मकाः कथं वा तेषां सत्यत्वमिति च वक्तव्य- मिति पञ्चभूतानां सत्यानां कार्यकारणात्मकानां स्वरूपाव- धारणार्थम् इदं ब्राह्मणमारभ्यते-
यदुपाधिविशेषापनय- द्वारेण 'नेति नेति' इति ब्रह्मणः सतत्त्वं निर्दिधारयि- षितम् । तत्र द्विरूपं ब्रह्म पञ्चभूतजनितकार्यकरणसंबद्धं मूर्तामूर्ताख्यं मर्त्यामृतस्वभावं तज्जनितवासनारूपं च सर्वज्ञं सर्वशक्ति सोपाख्यं भवति । क्रियाकारकफलात्मकं च स- र्वव्यवहारास्पदम् । तदेव ब्रह्म विगतसर्वोपाधिविशेषं सम्य- ग्दर्शनविषयम् अजरम् अमृतम् अभयम् , वाङ्मनसयोरप्य- विषयम् अद्वैतत्वात् 'नेति नेति' इति निर्दिश्यते । तत्र यदपोहद्वारेण 'नेति नेति' इति निर्दिश्यते ब्रह्म, ते एते द्वे वाव- वावशब्दोऽवधारणार्थः- द्वे एवेत्यर्थः- ब्रह्मणः परमात्मनः रूपे- रूप्यते याभ्याम् अरूपं परं ब्रह्म अविद्याध्यारोप्यमाणाभ्याम् । के ते द्वे? मूर्तं चैव मूर्तमेव च; तथा अमूर्तं च अमूर्तमेव चेत्यर्थः । अन्तर्णीतस्वात्मविशेषणे मूर्तामूर्ते द्वे एवेत्यवधार्येते; कानि पुनस्तानि विशेषणानि मूर्तामूर्तयोरित्युच्यन्ते- मर्त्यं च मर्त्यं मरणधर्मि, अमृतं च तद्विपरीतम्, स्थितं च-परिच्छिन्नं गतिपूर्वकं यत्तस्थास्नु, यच्च- यातीति यत्- व्यापि अपरिच्छिन्नं स्थितविपरीतम्, सच्च- सदित्यन्येभ्यो विशेष्यमाणासाधारणधर्मविशेषवत्, त्यच्च- तद्विपरीतम् 'यत्' इत्येव सर्वदा परोक्षाभिधानार्हम् ॥
In plain words
We have already said that the breaths are the real. Their mystic identification was explained earlier, in the course of discussing the teaching about Brahman — 'these very breaths are those' — and that much is settled. But what are they actually made of, and how do we know their reality holds up? That still needs saying. So, to pin down exactly what the five elements are — real things, each both cause and effect at once — this section now begins.
The whole point of stripping away every limiting label — the whole point of 'not this, not this' — is to pin down what is truly real about Brahman. Seen through the five elements and the whole world of cause and effect they build, Brahman shows up in two guises: one with form, one without, one mortal, one deathless, each carrying the imprint of the tendencies that world leaves behind; and in that guise it knows everything, can do anything, and even wears a name. It is every action, every actor, every result — the ground every transaction stands on. But that same Brahman, once every one of those labels falls away, is simply the object of clear seeing: ageless, deathless, fearless, beyond anything speech or mind can reach, because there is nothing second to it — and that is exactly why it can only be pointed at by saying what it isn't, twice over. Now, the Brahman pointed at that way, by pure negation — these two we've just named are exactly it. ('Indeed' here just means: no more, no less than two.) Call them Brahman's two forms because it's through these two — laid over the formless, supreme Brahman by nothing but ignorance — that the formless gets a face at all. And what are the two? Simply this: the formed, exactly the formed; the formless, exactly the formless. Each of those carries its own further qualities, but there are still only two main kinds. So what are those qualities? Mortal means bound to die; immortal is just the opposite. Fixed means limited — something that has stopped moving and now just stands. 'Yat' comes from 'it goes,' so it means the opposite: unbounded, everywhere. 'Sat' names whatever has some distinguishing feature that sets it apart from everything else. And 'tyat' is the opposite of that — like 'yat,' a thing that can only ever be pointed at from a distance, never named outright.
Close to the Sanskrit
There, it has been said that the breaths, indeed, are the real. The mystic teachings of the breaths, which were explained in connection with the topic of the Brahman-teaching — 'these are those very breaths' — thus too. Of what nature are they, or how is their reality to be established? — this still needs to be said. Thus, for the purpose of ascertaining the true nature of the five elements, which are real and consist of cause and effect, this section is begun.
That the true nature of what is real belonging to Brahman is intended to be firmly ascertained by means of removing specific limiting adjuncts, through 'not this, not this' — [this has been the aim]. There, Brahman, twofold in form, bound up with the effect-and-cause produced from the five elements, called 'formed' and 'formless', of mortal-and-immortal nature, and taking the shape of the latent impressions produced from that, is omniscient, omnipotent, and possesses a designation. And it consists of action, agent, and result, and is the ground of every worldly transaction. That very Brahman, stripped of every specific limiting adjunct, the object of right seeing, ageless, immortal, fearless, beyond the reach even of speech and mind because of its non-duality, is designated 'not this, not this.' There, the Brahman that is designated 'not this, not this' by the process of negation — these are those very two, indeed. (The word 'vāva' carries the sense of emphasis — the meaning is 'exactly two.') The two forms of Brahman, the Supreme Self — called forms because through these two, superimposed by nescience, the formless supreme Brahman is given a shape. What are those two? The formed, precisely the formed; and likewise the formless, precisely the formless — that is the meaning. The formed and the formless, each carrying its own further qualifying attributes, are ascertained to be exactly two. What, again, are those attributes of the formed and the formless? — they are now stated: 'mortal' means subject to the property of dying; 'immortal' is the opposite of that. 'Fixed' means limited, that which, having previously moved, has come to stand still. And 'yat' — from 'yāti,' it goes, hence 'yat' — means all-pervading, unlimited, the opposite of the fixed. And 'sat' — the word 'sat' denotes something possessing a specific uncommon property that distinguishes it from other things. And 'tyat' is the opposite of that; like 'yat,' it is always fit only for indirect designation.