तद्यथा तृणजलायुका तृणस्यान्तं गत्वान्यमाक्रममाक्रम्यात्मानमुपसंहरति ।एवमेवायमात्मेदं शरीरं निहत्याविद्यां गमयित्वान्यमाक्रममाक्रम्यात्मानमुपसंहरति ॥४,४.३ ॥
In plain words
As a caterpillar, having come to the end of a blade of grass, draws itself together after reaching for another, so this self, having struck down this body and dispelled its ignorance, draws itself together after reaching for another.
The commentators
Bṛhadāraṇyakopaniṣadbhāṣyam· Leaf 9.273· avatāraṇikā
एवं विद्यादिसंभारसंभृतो देहान्तरं प्रतिपद्यमानः, मुक्त्वा पूर्वं देहम्, पक्षीव वृक्षान्तरम्, देहान्तरं प्रतिपद्यते ; अथवा आतिवाहिकेन शरीरान्तरेण कर्मफलजन्मदेशं नीयते । किंचात्रस्थस्यैव सर्वगतानां करणानां वृत्तिलाभो
In plain words
So the self, carrying everything it has gathered — its knowledge, its actions, its old habits of mind — lets go of this body the way a bird leaves one tree for another, and takes up a new one. Or, some say, it travels there in a subtle carrier-body suited to where its actions are sending it. But then a further question opens up: if the senses already reach everywhere, does something at the new place gain the use of them even without having to travel...
Close to the Sanskrit
Thus, one who has gathered the provisions beginning with knowledge, while attaining another body — having abandoned the former body, like a bird [going to] another tree, attains another body; or else he is led by a transitional body to the place of birth [determined by] the fruit of action. Moreover, is there an attainment of function of the organs, which are all-pervading, even for one remaining right here...
Bṛhadāraṇyakopaniṣadbhāṣyam· Leaf 9.274· bhāṣya
भवति, आहोस्वित् शरीरस्थस्य संकुचितानि करणानि मृत- स्य भिन्नघटप्रदीपप्रकाशवत् सर्वतो व्याप्य पुनः देहान्तरा- रम्भे संकोचमुपगच्छन्ति- किंच मनोमात्रं वैशेषिकसमय इव देहान्तरारम्भदेशं प्रति गच्छति, किं वा कल्पमान्तरमेव वेदान्तसमये- इत्युच्यते-- 'त एते सर्व एव समाः सर्वे- ऽनन्ताः' इति श्रुतः सर्वात्मकानि तावत्करणानि, सर्वात्म- कप्राणसंश्रयाच्च ; तेषाम् आध्यात्मिकाधिभौतिकपरिच्छेदः प्राणिकर्मज्ञानभावनानिमित्तः ; अतः तद्वशात् स्वभावतः सर्व- गतानामनन्तानामपि प्राणानां कर्मज्ञानवासनानुरूपेणैव दे- हान्तरारम्भवशात् प्राणानां वृत्तिः संकुचति विकसति च ; तथा चोक्तम् 'समः प्लुषिणा समो मशकेन समो नागेन सम एभिस्त्रिभिर्लोकैः समोऽनेन सर्वेण' इति ; तथा च इदं वचनमनुकूलम्--' स यो हैताननन्तानुपास्ते' इत्यादि, 'तं यथा यथोपासते' इति च । तत्र वासना पूर्वप्रज्ञाख्या वि- द्याकर्मतन्त्रा जलूकावत् संततैव स्वप्रकाश इव कर्मकृतं देहा- द्देहान्तरम् आरभते हृदयस्थैव ; पुनर्देहान्तरारम्भे देहान्तरं पूर्वाश्रयं विमुञ्चति- इत्येतस्मिन्नर्थे दृष्टान्त उपादीयते-
In plain words
So which is it: do a dead person's organs — normally packed tightly into one body — suddenly spread everywhere, the way a lamp's light fills a room once its covering jar shatters, and only then contract again into the next body? Or, as some philosophers hold, does the mind alone travel to where the new body is starting, and Vedānta actually has an entirely different picture? Here is the answer, straight from scripture: these faculties are all equal, all infinite, every one of them. On their own they are already everywhere, resting as they do on a breath that is itself everywhere. What seems to divide 'my organs' from 'the world out there' is not any real boundary in them — it comes entirely from the impressions built up by what a being does and knows. So even though these faculties are, by nature, boundless, their actual working range tightens or opens up strictly according to that impression, and according to whatever new body is beginning. Hence the old teaching: equal to a gnat, equal to a mosquito, equal to an elephant, equal to these three worlds, equal to the whole of everything — and, in the same spirit, whatever you meditate on, that is exactly what you become. The real mover, then, is a single impression — the one called 'prior cognition,' shaped entirely by knowledge and action, continuous like the leech, glowing as if with its own light, and never actually leaving the heart even as it carries you from this body into the next, and then lets that support go once the new one is taken up. To make this vivid, here comes an illustration.
Close to the Sanskrit
Does it occur, or else do the organs of one who has died — [ordinarily] contracted while confined to a body — having pervaded everywhere like the light of a lamp [freed from] a broken pot, again undergo contraction at the commencement of another body? Moreover, does mind alone go toward the place where another body commences, as in the Vaiśeṣika doctrine; or is it, rather, an altogether different scheme according to the Vedānta doctrine? — To this it is answered: 'these, all of them, are equal, all infinite' — thus from scripture. The organs, to begin with, are all-pervading in nature, and also because of their dependence on the all-pervading vital breath. Their division into the individual and the elemental has as its cause the dispositions arising from a living being's action and knowledge. Therefore, owing to that, by their own nature, of the vital faculties — even though all-pervading and infinite — precisely in accordance with the impression left by action and knowledge, and owing to the commencement of a [new] body, the function of the vital faculties contracts and expands. And so it has been said: 'equal to a gnat, equal to a mosquito, equal to an elephant, equal to these three worlds, equal to this whole' — thus. And likewise this statement agrees: 'he who meditates on these infinite ones,' etc., and 'as, in whatever way, they meditate' — thus. There, the impression called 'prior cognition,' governed by knowledge and action, continuous like the leech, self-luminous as it were, brought about by action, initiates [the passage] from [one] body to another body, remaining all the while in the heart; again, at the commencement of [yet] another body, it releases the [former] body that was its previous support. — On this very point an illustration is now adduced—
Bṛhadāraṇyakopaniṣadbhāṣyam· Leaf 9.275· bhāṣya
तत् तत्र देहान्तरसंचारे इदं निदर्शनम्- यथा येन प्रकारेण तृणजलायुका तृणजलूका तृणस्य अन्तम् अवसा- नम्, गत्वा प्राप्य, अन्यं तृणान्तरम्, आक्रमम्- आक्रम्यत इत्याक्रमः- तमाक्रमम्, आक्रम्य आश्रित्य, आत्मानम् आत्मनः पूर्वावयवम् उपसंहरति अन्त्यावयवस्थाने; एवमेव अयमात्मा यः प्रकृतः संसारी इदं शरीरं पूर्वोपात्तम्, नि- हत्य स्वप्नं प्रतिपित्सुरिव पातयित्वा अविद्यां गमयित्वा अचे- तनं कृत्वा स्वात्मोपसंहारेण, अन्यम् आक्रमम् तृणान्तरमिव तृणजलूका शरीरान्तरम्, गृहीत्वा प्रसारितया वासनया, आत्मानमुपसंहरति, तत्र आत्मभावमारभते- यथा स्वप्ने देहान्तरस्थ एव शरीरारम्भदेशे- आरभ्यमाणे देहे जङ्गमे स्थावरे वा । तत्र च कर्मवशात् करणानि लब्धवृत्तीनि संह- न्यन्ते; बाह्यं च कुशमृत्तिकास्थानीयं शरीरमारभ्यते; तत्र च करणव्यूहमपेक्ष्य वागाद्यनुग्रहाय अग्न्यादिदेवताः संश्र- यन्ते । एष देहान्तरारम्भविधिः ॥
In plain words
Here is the illustration behind that verse. Picture the grass-leech exactly as described: it reaches the very tip of its blade — that word just means the point where the blade runs out — and takes hold of another one. An 'ākrama' is simply defined as whatever you can step onto. Having secured that new hold, the leech draws its trailing end up to where its front end already is. That is exactly what you are doing at death. You let this old, long-held body fall — the way someone about to slip into a dream lets waking life go — you release the ignorance bound up with it, you drain it of awareness, and by withdrawing your very self from it you reach for a new foothold, the way the leech reaches for new grass, and that foothold is another body. You take hold of it through nothing more than your own stretched-out impressions, draw yourself fully onto it, and right there the sense 'I am this' begins — just as, in a dream, you are already living in another body even while it is still forming, whether that body will move about or stand rooted like a plant. Meanwhile, driven purely by karma, your organs — each already carrying its own capacity — gather themselves together, and outwardly a body starts to take shape from what is, in truth, no finer than grass and clay. And once that array of organs is in place, the presiding powers — Agni and the rest — come to lend speech and the other faculties their support. That, in full, is how a new body begins.
Close to the Sanskrit
That [verse], in this connection regarding the passage from one body to another, is an illustration. That is: in whatever manner the grass-leech — the grass-leech — having gone to, [that is,] reached, the end, [that is,] the termination, of the grass, [steps upon] another blade of grass — an ākrama [is defined as] that which is stepped upon, thus [is] an ākrama — having stepped upon, [that is,] taken as a support, that foothold, draws its own prior part in at the place of [its] final part — exactly so does this self, who is the transmigrator here under discussion, having struck down this body [which was] previously acquired, like one about to enter a dream, having cast [it] down, having dispelled ignorance, having rendered [it] insentient, by the withdrawal of its own self, take another foothold — like another blade of grass to the grass-leech — [namely] another body, having seized [it] by means of its extended impression, draws itself in, and there begins the notion of selfhood — just as, in a dream, one is already situated in another body even while the body, moving or stationary, is being formed at the place where [that] body is to begin. There, moreover, owing to karma, the organs, having [each] obtained their function, are assembled together; and outwardly the body, standing in the place of grass and clay, is initiated; and there, having regard to the assemblage of organs, the deities beginning with Agni resort [to it] for the support of speech and the rest. This is the rule for the commencement of another body.