In plain words
A person does not attain freedom from action merely by not beginning actions, nor does one reach perfection merely by sannyāsa alone.
Word for word
Not from not undertaking actions does a person attain naiṣkarmya, actionlessness, nor by renunciation alone does [he] reach perfection.
The commentators
Bhagavadgītābhāṣyam
‘मां च बन्धकारणे कर्मण्येव नियोजयसि’ इति विषण्णमनसमर्जुनम् ‘कर्म नारभे’ इत्येवं मन्वानमालक्ष्य आह भगवान् — न कर्मणामनारम्भात् इति । अथवा — ज्ञानकर्मनिष्ठयोः परस्परविरोधात् एकेन पुरुषेण युगपत् अनुष्ठातुमशक्त्यत्वे सति इतरेतरानपेक्षयोरेव पुरुषार्थहेतुत्वे प्राप्ते कर्मनिष्ठाया ज्ञाननिष्ठाप्राप्तिहेतुत्वेन पुरुषार्थहेतुत्वम् , न स्वातन्त्र्येण ; ज्ञाननिष्ठा तु कर्मनिष्ठोपायलब्धात्मिका सती स्वातन्त्र्येण पुरुषार्थहेतुः अन्यानपेक्षा, इत्येतमर्थं प्रदर्शयिष्यन् आह भगवान् —
न कर्मणां क्रियाणां यज्ञादीनाम् इह जन्मनि जन्मान्तरे वा अनुष्ठितानाम् उपात्तदुरितक्षयहेतुत्वेन सत्त्वशुद्धिकारणानां तत्कारणत्वेन च ज्ञानोत्पत्तिद्वारेण ज्ञाननिष्ठाहेतूनाम् , ‘ज्ञानमुत्पद्यते पुंसां क्षयात्पापस्य कर्मणः । यथादर्शतलप्रख्ये पश्यत्यात्मानमात्मनि’(मो. ध. २०४ । ८) इत्यादिस्मरणात् , अनारम्भात् अननुष्ठानात् नैष्कर्म्यं निष्कर्मभावं कर्मशून्यतां ज्ञानयोगेन निष्ठां निष्क्रियात्मस्वरूपेणैव अवस्थानमिति यावत् । पुरुषः न अश्नुते न प्राप्नोतीत्यर्थः ॥
कर्मणामनारम्भान्नैष्कर्म्यं नाश्नुते इति वचनात् तद्विपर्ययात् तेषामारम्भात् नैष्कर्म्यमश्नुते इति गम्यते । कस्मात् पुनः कारणात् कर्मणामनारम्भान्नैष्कर्म्यं नाश्नुते इति ? उच्यते, कर्मारम्भस्यैव नैष्कर्म्योपायत्वात् । न ह्युपायमन्तरेण उपेयप्राप्तिरस्ति । कर्मयोगोपायत्वं च नैष्कर्म्यलक्षणस्य ज्ञानयोगस्य, श्रुतौ इह च, प्रतिपादनात् । श्रुतौ तावत् प्रकृतस्य आत्मलोकस्य वेद्यस्य वेदनोपायत्वेन ‘तमेतं वेदानुवचनेन ब्राह्मणा विविदिषन्ति यज्ञेन’(बृ. उ. ४ । ४ । २२) इत्यादिना कर्मयोगस्य ज्ञानयोगोपायत्वं प्रतिपादितम् । इहापि च — ‘संन्यासस्तु महाबाहो दुःखमाप्तुमयोगतः’(भ. गी. ५ । ६) ‘योगिनः कर्म कुर्वन्ति सङ्गं त्यक्त्वात्मशुद्धये’(भ. गी. ५ । ११) ‘यज्ञो दानं तपश्चैव पावनानि मनीषिणाम्’(भ. गी. १८ । ५) इत्यादि प्रतिपादयिष्यति ॥
ननु च ‘अभयं सर्वभूतेभ्यो दत्त्वा नैष्कर्म्यमाचरेत्’(अश्व. ४६ । १८) इत्यादौ कर्तव्यकर्मसंन्यासादपि नैष्कर्म्यप्राप्तिं दर्शयति । लोके च कर्मणामनारम्भान्नैष्कर्म्यमिति प्रसिद्धतरम् । अतश्च नैष्कर्म्यार्थिनः किं कर्मारम्भेण ? इति प्राप्तम् । अत आह — न च संन्यसनादेवेति । नापि संन्यसनादेव केवलात् कर्मपरित्यागमात्रादेव ज्ञानरहितात् सिद्धिं नैष्कर्म्यलक्षणां ज्ञानयोगेन निष्ठां समधिगच्छति न प्राप्नोति ॥ ४ ॥
In plain words
Kṛṣṇa noticed Arjuna's mind sinking under a thought he hadn't spoken aloud — 'you're pushing me into action, the very thing that binds me' — and a resolve forming behind it: 'then I simply won't act.' Seeing this, Kṛṣṇa begins: 'not by refusing to undertake action...' Or, put another way: knowledge and action pull against each other so directly that no one person can practice both at once. And yet each has already been shown to lead, on its own, to a genuine human good. So how does action serve that good? Only indirectly — by leading to knowledge. It has no independent power of its own to do so. Knowledge, on the other hand, even though it only comes into being through the discipline of action as its means, stands entirely on its own once it has arisen — it needs nothing else to accomplish the human good. It is exactly this distinction Kṛṣṇa is about to make clear when he speaks next —
This isn't about actions themselves — rites like sacrifice and the rest, done in this life or another. Such rites do serve a purpose: they burn away accumulated sin, and that purifies the mind, and a purified mind opens, in turn, into knowledge — so these rites are, indirectly, causes of the discipline of knowledge. As it is said: 'knowledge arises in a person once sinful action is destroyed, and then one sees the Self within the self, clear as in a mirror's face' (Mokṣadharma 204.8) — this is what such traditional texts tell us. So 'from non-undertaking' simply means from not performing action at all; and naiṣkarmya means exactly this: the state of being without action, action's own emptiness, the discipline reached through the yoga of knowledge — which is nothing other than abiding as the actionless Self itself. That's the whole of what it means. And a person does not reach that state — does not obtain it — merely by refusing to act.
From the statement 'one does not attain actionlessness merely by not undertaking action,' the reverse follows naturally: one attains actionlessness precisely by undertaking action. But why should that be true? Here is the answer: undertaking action is itself the means to actionlessness — and there is simply no reaching a goal without a means. This is exactly what scripture establishes, both in the Veda and here in the Gītā: that karma-yoga is the means to jñāna-yoga, the yoga marked by actionlessness. In scripture, first of all, we read: 'this very Self the brāhmaṇas seek to know through the study of the Veda, through sacrifice' (Bṛhadāraṇyaka Upaniṣad 4.4.22) — because these are precisely the means of coming to know that Self under discussion. And the Gītā itself will say the same thing, again and again: 'renunciation, mighty-armed one, is hard to reach without yoga' (BG 5.6); 'yogins act, having let go of attachment, purely to purify themselves' (BG 5.11); 'sacrifice, giving, and austerity all purify the wise' (BG 18.5) — Kṛṣṇa will make this very point himself, later on.
But someone might object: doesn't scripture itself say, in a passage like 'having given fearlessness to all beings, one should take up actionlessness' (Aśvamedhika[parva] 46.18), that even simply renouncing your obligatory duties gets you to actionlessness? And in everyday life too, isn't it common wisdom that actionlessness comes precisely from not doing anything? So why, the objector asks, should someone who wants actionlessness bother undertaking action at all? That's the objection as it stands. And to this Kṛṣṇa replies: 'and not by renunciation alone' — that's how the verse begins. Because renunciation by itself, mere abandonment of action with no knowledge behind it, never actually gets you to that accomplishment marked by actionlessness, the discipline reached through the yoga of knowledge. It simply does not get you there.
Close to the Sanskrit
Perceiving Arjuna, whose mind was dejected with the thought, '[you] engage me in action alone, which is the cause of bondage', thinking thus, 'I will not undertake action' — the Lord spoke: 'not through the non-undertaking of actions' — thus [begins the verse]. Or, alternatively — because of the mutual incompatibility of the two disciplines of knowledge and action, there being an inability for a single person to practice [both] simultaneously, [and] it having been established that each, independently of the other, is a cause of a human end — the status of the discipline of action as a cause of a human end is by virtue of its being the cause of attaining the discipline of knowledge, not through its own independence; whereas the discipline of knowledge, though its very being is attained through the means of the discipline of action, once so attained, is independently a cause of the human end, not dependent on anything else — intending to demonstrate this very point, the Lord spoke —
Not of actions — [i.e.] of rites such as sacrifices and so on, performed either in this birth or in another birth — [these being] causes of the destruction of accumulated sin, [and thereby] causes of the purity of one's inner being, and, by virtue of being the cause of that [purity], through the gateway of the arising of knowledge, causes of the discipline of knowledge — 'knowledge arises for people from the destruction of sinful action; [then] one sees the Self in the self, as in the surface of a mirror' (Mokṣadharma 204.8) — because of such traditional statements: 'from non-undertaking' [means] from non-performance; naiṣkarmya [means] the state of being without action, the emptiness of action, the discipline by the yoga of knowledge, [i.e.] abiding precisely in the nature of the actionless Self — that much [is the meaning]. A person does not attain, does not obtain [it this way]. Such is the meaning.
From the statement 'from the non-undertaking of actions, [one] does not attain naiṣkarmya', by the reverse of that, it is understood: 'from the undertaking of them, [one] attains naiṣkarmya.' But for what reason [is it that] 'from the non-undertaking of actions, [one] does not attain naiṣkarmya'? It is said [in answer]: because the undertaking of action itself is the means to naiṣkarmya. For indeed, without a means, there is no attainment of the goal. And [this is known] because of the establishment, in scripture and here [in the Gītā] as well, of the status of karma-yoga as the means to jñāna-yoga, characterized by naiṣkarmya. In scripture, first, by [the statement] 'this very [Self] the brāhmaṇas seek to know through the recitation of the Veda, through sacrifice' (Bṛhadāraṇyaka Upaniṣad 4.4.22) and so on — by virtue of being the means of knowing that Self-world under discussion, which is to be known — the status of karma-yoga as the means to jñāna-yoga is established. And here too [in the Gītā] — 'but renunciation, O mighty-armed one, is difficult to attain without yoga' (BG 5.6); 'yogins perform action, having abandoned attachment, for the purification of the self' (BG 5.11); 'sacrifice, gift, and austerity too are purifying for the wise' (BG 18.5) — and so on, he will establish [this same point].
But, [an objector says,] in [the passage] 'having given fearlessness to all beings, one should practice actionlessness' (Aśvamedhika[parva] 46.18) and such, [scripture] shows the attainment of actionlessness even through the renunciation of obligatory action. And in ordinary usage too, [the notion] 'actionlessness [comes] from the non-undertaking of actions' is even more well-established. And therefore, for one who desires actionlessness, what [use is there] in undertaking action? Thus [this objection] has been arrived at. In response to this, [Kṛṣṇa] says — 'and not by renunciation alone' — thus [begins the reply]. Nor, indeed, from renunciation alone, [i.e.] from the mere abandonment of action alone, devoid of knowledge, does one attain the accomplishment characterized by actionlessness, the discipline by the yoga of knowledge — [i.e.] does not obtain [it].