In plain words
That which is night to all beings is wakefulness to the self-controlled; and what beings are awake to is night to the sage who truly sees.
Word for word
What is night for all beings, in that the self-controlled one is awake; that in which beings are awake is night for the sage who sees.
The commentators
Bhagavadgītābhāṣyam
योऽयं लौकिको वैदिकश्च व्यवहारः स उत्पन्नविवेकज्ञानस्य स्थितप्रज्ञस्य अविद्याकार्यत्वात् अविद्यानिवृत्तौ निवर्तते, अविद्यायाश्च विद्याविरोधात् निवृत्तिः, इत्येतमर्थं स्फुटीकुर्वन् आह —
या निशा रात्रिः सर्वपदार्थानामविवेककरी तमःस्वभावत्वात् सर्वभूतानां सर्वेषां भूतानाम् । किं तत् परमार्थतत्त्वं स्थितप्रज्ञस्य विषयः । यथा नक्तञ्चराणाम् अहरेव सदन्येषां निशा भवति, तद्वत् नक्तञ्चरस्थानीयानामज्ञानां सर्वभूतानां निशेव निशा परमार्थतत्त्वम् , अगोचरत्वादतद्बुद्धीनाम् । तस्यां परमार्थतत्त्वलक्षणायामज्ञाननिद्रायाः प्रबुद्धो जागर्ति संयमी संयमवान् , जितेन्द्रियो योगीत्यर्थः । यस्यां ग्राह्यग्राहकभेदलक्षणायामविद्यानिशायां प्रसुप्तान्येव भूतानि जाग्रति इति उच्यन्ते, यस्यां निशायां प्रसुप्ता इव स्वप्नदृशः, सा निशा अविद्यारूपत्वात् परमार्थतत्त्वं पश्यतो मुनेः ॥
अतः कर्माणि अविद्यावस्थायामेव चोद्यन्ते, न विद्यावस्थायाम् । विद्यायां हि सत्याम् उदिते सवितरि शार्वरमिव तमः प्रणाशमुपगच्छति अविद्या । प्राक् विद्योत्पत्तेः अविद्या प्रमाणबुद्ध्या गृह्यमाणा क्रियाकारकफलभेदरूपा सती सर्वकर्महेतुत्वं प्रतिपद्यते । न अप्रमाणबुद्ध्या गृह्यमाणायाः कर्महेतुत्वोपपत्तिः, ‘प्रमाणभूतेन वेदेन मम चोदितं कर्तव्यं कर्म’ इति हि कर्मणि कर्ता प्रवर्तते, न ‘अविद्यामात्रमिदं सर्वं निशेव’ इति । यस्य पुनः ‘निशेव अविद्यामात्रमिदं सर्वं भेदजातम्’ इति ज्ञानं तस्य आत्मज्ञस्य सर्वकर्मसंन्यासे एव अधिकारो न प्रवृत्तौ । तथा च दर्शयिष्यति — ‘तद्बुद्धयस्तदात्मानः’(भ. गी. ५ । १७) इत्यादिना ज्ञाननिष्ठायामेव तस्य अधिकारम् ॥
तत्रापि प्रवर्तकप्रमाणाभावे प्रवृत्त्यनुपपत्तिः इति चेत् , न ; स्वात्मविषयत्वादात्मविज्ञानस्य । न हि आत्मनः स्वात्मनि प्रवर्तकप्रमाणापेक्षता, आत्मत्वादेव । तदन्तत्वाच्च सर्वप्रमाणानां प्रमाणत्वस्य । न हि आत्मस्वरूपाधिगमे सति पुनः प्रमाणप्रमेयव्यवहारः सम्भवति । प्रमातृत्वं हि आत्मनः निवर्तयति अन्त्यं प्रमाणम् ; निवर्तयदेव च अप्रमाणीभवति, स्वप्नकालप्रमाणमिव प्रबोधे । लोके च वस्त्वधिगमे प्रवृत्तिहेतुत्त्वादर्शनात् प्रमाणस्य । तस्मात् न आत्मविदः कर्मण्यधिकार इति सिद्धम् ॥ ६९ ॥
In plain words
All this activity — both worldly and ritual — is simply a product of ignorance. So for someone of steady wisdom, in whom discriminating knowledge has already dawned, this activity naturally falls away once ignorance itself has ceased. And ignorance ceases because knowledge directly opposes it. Making this point clear, Śaṅkara now says —
'Night' here means whatever, by its very nature of darkness, blinds every being to the discrimination of things. And what is that, for someone of steady wisdom? It is the truth of ultimate reality itself — that is his true object of awareness. Consider nocturnal creatures: for them, ordinary daylight, though it is daylight to others, is effectively night. In just the same way, for all ordinary, ignorant beings — who are really playing the role of nocturnal creatures here — the truth of ultimate reality is like night, simply because it lies completely beyond the reach of minds not attuned to it. Someone awakened from that sleep of ignorance about ultimate reality — the self-controlled one, the one who has conquered his senses, the yogi — is the one who is truly 'awake.' That is the meaning. In this night of ignorance, structured around the split between what is perceived and who perceives it, beings who are actually fast asleep call themselves 'awake' — asleep like dreamers lost inside their own dream. And that very state — the ordinary waking state everyone else takes for granted — is, for the sage who actually sees ultimate reality, nothing but night, because it is itself simply a form of ignorance.
That is why actions are prescribed only in the state of ignorance, never in the state of knowledge. Once knowledge is present, once the sun has risen, ignorance simply dissolves, the way night's darkness dissolves at sunrise. Before knowledge arises, ignorance — taken as valid by an ordinary, practical way of knowing, structured around the difference between an action, its agent, and its result — plays the role of causing all action. But if that same ignorance were grasped as invalid, there would be no coherent way for it to cause action at all; because someone actually acts thinking, 'this action is enjoined on me by the Veda, which I trust as valid, and it must be done' — never thinking, 'all of this is just ignorance, like a night.' But someone who does have that knowledge — 'all this multiplicity of distinctions is just ignorance, like a mere night' — that knower of the Self belongs only with the renunciation of all action, not with continuing to act. And Kṛṣṇa will go on to show exactly this: in the verse 'those whose understanding, whose very self, is fixed on That...' (5.17) and what follows, he shows that such a person's place is only in abiding in knowledge.
Someone might object: but even so, without some proof that impels you toward it, how could a knower's own state of being simply carry on? No — because Self-knowledge has one's own Self as its object. The Self needs no external proof to impel it toward its own nature, for the simple reason that it already is the Self; and every means of knowledge ultimately terminates in it anyway. Once you have actually reached the Self's own true nature, the whole business of knower-and-known, proof-and-proven, no longer applies. In fact, that final proof of Self-knowledge cancels out the very status of being 'a knower' belonging to the Self — and in the very act of cancelling it, that proof itself stops being a valid proof, the way what counted as knowledge inside a dream stops counting once you wake up. And in ordinary life, we never see a proof go on causing further activity once the thing it was proving has already been established. So it is proven: the knower of the Self has no entitlement to action.
Close to the Sanskrit
This worldly and Vedic activity, since it is an effect of ignorance, ceases, for one of steady wisdom in whom discriminating knowledge has arisen, upon the cessation of ignorance; and the cessation of ignorance itself [follows] because knowledge is opposed to it — making this point clear, he [Śaṅkara] says —
'Which is night' — night — that obscures discrimination regarding all things, because of its very nature of darkness, for all beings, for all creatures. What is that? It is the truth of ultimate reality, the object [of awareness] of one of steady wisdom. Just as, for nocturnal creatures, day itself, though it is [day], becomes night for others — in the same way, for all ignorant beings who play the role of nocturnal creatures, the truth of ultimate reality is like a night, because it lies beyond the range [of perception] of those whose understanding is not of that. Awakened from that sleep of ignorance which is characterized as the truth of ultimate reality, the self-controlled one, possessing self-control, one who has conquered the senses, the yogi, 'is awake': such is the meaning. In that night of ignorance, characterized by the distinction of the graspable and the grasper, beings who are indeed asleep are said to be 'awake'; in that night, [ordinary beings are] asleep, like dreamers — that [ordinary waking state] is [itself] night, because it is a form of ignorance, for the sage who sees the truth of ultimate reality.
Therefore, actions are enjoined only in the state of ignorance, not in the state of knowledge. For when knowledge is present, when the sun has risen, ignorance goes to destruction, like nocturnal darkness. Before the arising of knowledge, ignorance, grasped by a [merely practical, not ultimately valid] cognition, having the form of the distinction between action, factor, and result, assumes the role of the cause of all action. There is no tenability in ignorance's being the cause of action if it is grasped by an invalid cognition; for the agent engages in action [thinking,] 'action, enjoined for me, to be done, by the Veda which serves as a valid means' — not [thinking,] 'all this is mere ignorance, like a night.' But for one who has the knowledge 'like a night, all this is mere ignorance, this whole multiplicity of distinctions,' for that knower of the Self, entitlement lies only in the renunciation of all action, not in engagement [in action]. And accordingly he will show, by [the verse] 'those whose understanding, whose self, is fixed on That' (Gītā 5.17) and so on, that his entitlement lies only in the grounding of knowledge.
If it be objected that, even so, in the absence of an impelling means of valid knowledge, [the knower's own] engagement [in anything] is untenable — no; because Self-knowledge has one's own Self as its object. For the Self has no dependence, with regard to its own nature, on an impelling means of valid knowledge, simply because it is [already] the Self; and because the validity of all means of valid knowledge culminates in that [Self]. For, once the Self's own nature has been attained, the transactional relation of valid means and validated object is no longer possible. For the final means of valid knowledge removes the [very] status of being a knower belonging to the Self; and in the very act of negating it, it itself becomes invalid — like the valid knowledge of the dream-state, upon waking. And in the world, a means of valid knowledge is never observed to be a cause of activity once the thing has [already] been ascertained. Therefore it is established that there is no entitlement to action for the knower of the Self. [End of commentary on verse 69.]
Gītā Vāhini
“ఇంద్రియములను సర్వతోముఖంగా నిగ్రహించుట స్థితప్రజ్ఞ లక్షణము.” అన్ని భూతములకూ ఏది రాత్రియో స్థితప్రజ్ఞుడు అందులో మేల్కొనియుండును. అన్ని భూతములు మేల్కొనియున్న కాలములో స్థితప్రజ్ఞుడు నిద్రించుచుండును. (అనగా పగలు రాత్రిగను, రాత్రి పగలుగనూ) దీనికి సామాన్యముగా లోకమున అక్షరార్థమును తీసుకొనుచున్నారు. అది స్థితప్రజ్ఞుని లక్షణమనకు సరితూగదు. అంటే, సామాన్య మానవులు రాత్రి నిద్రించు సమయమున స్థితప్రజ్ఞుడు మేలుకొనుననియు, సామాన్య మానవులు మేలుకొనిన పగలు స్థితప్రజ్ఞునికి రాత్రి అనియు తలంతురు. దీనికి గూఢార్థము అతి సున్నితము. లౌకిక విషయములందు సామాన్య మానవులు జాగ్రత వహించుయుందురు. అనగా విషయేంద్రియములందు విజృంభించి యుండుటే మేలుకొనుట. కాని, స్థితప్రజ్ఞుడు ఇట్టి విషయేంద్రియములందు విముఖుడై యుండును. అనగా నిద్రించును. విషయేంద్రియములను అరికట్టుటే నిద్రించుట. విషయములను రేకెత్తించుటే మేలుకొనుట. సామాన్య మానవులు ఇంద్రియములను విజృంభింపచేయునపుడు స్థితప్రజ్ఞుని ఇంద్రియములు నిద్రించును. దీనినే, మరొక రీతిగా కూడనూ చెప్పవచ్చును. ఆత్మస్థితిని మరచుట, దేహస్థితిని పొందుట. ఇవి సామాన్య మానవుల స్థితులు. సామాన్య మానవులు ఆత్మస్థితి యందు మరచి నిద్రించుచూ దేహస్థితిలో మేల్కొనుచుందురు. స్థితప్రజ్ఞుండు అట్లుకాక ఆత్మస్థితిలో మేల్కొని దేహస్థితిలో నిద్రించును. ఇదే “స్థితప్రజ్ఞ స్థితి”. సామాన్య మానవులు అజాగ్రత్తలో ఉన్న ఆత్మస్థితిలో స్థితప్రజ్ఞుడు జాగ్రతతోనూ, సామాన్యులు జాగ్రతలో ఉన్న విషయవాసనలలో స్థితప్రజ్ఞుడు అజాగ్రత్తతోను మెలగును. ఇదియే సూక్ష్మార్థము. కానీ, అందరూ మేలుకొన్న సమయమున స్థితప్రజ్ఞుడు నిద్రించుననియు అందరూ నిద్రించు సమయమున స్థితప్రజ్ఞుడు మేలుకొనుననియు అక్షరార్థమునే తెలిపిన దొంగలు, ఘూర్కాలు, పహారావారు ఇట్టివారందరూ రాత్రులందు మేల్కొని పగలు నిద్రింతురు కదా? వీరందరూ స్థితప్రజ్ఞులు కావలసినదేనా?
In plain words
"To control the senses in every direction is the mark of the sthitaprajña." What is night for all beings, in that the sthitaprajña stays awake. When all beings are awake, the sthitaprajña is asleep. (That is, day is night to him, and night is day.) Commonly in the world people take this in its literal meaning, but that does not measure up to the mark of the sthitaprajña. That is, they think that when ordinary people sleep at night the sthitaprajña is awake, and that the day, when ordinary people are awake, is night for the sthitaprajña. The hidden meaning of this is very subtle. Ordinary people stay wakeful in worldly matters. That is, to run rampant in the senses among their objects is what waking means. But the sthitaprajña stays turned away from such senses and their objects; that is, he sleeps. To restrain the senses among their objects is sleeping; to stir up sense objects is waking. When ordinary people let their senses run rampant, the sthitaprajña's senses sleep. This can also be put another way. Forgetting the state of the Ātma and taking on the state of the body: these are the states of ordinary people. Ordinary people forget, and sleep in the state of the Ātma, and are awake in the state of the body. The sthitaprajña is not like that: he is awake in the state of the Ātma and asleep in the state of the body. This is "the state of the sthitaprajña". In the state of the Ātma, where ordinary people are unwakeful, the sthitaprajña moves wakefully; among the vāsanās for sense objects, where ordinary people are wakeful, the sthitaprajña moves unwakefully. This is the subtle meaning. But if one gives only the literal meaning, that the sthitaprajña sleeps when everyone is awake and wakes when everyone sleeps, then thieves, Gurkhas, watchmen and all such people stay awake through the nights and sleep by day, do they not? Must all of them be sthitaprajñas, then?