In plain words
For Puruṣa, dwelling in Prakṛti, experiences the guṇas born of Prakṛti; attachment to those guṇas is the cause of its birth in good and bad wombs.
Word for word
For puruṣa, abiding in prakṛti, experiences the guṇas born of prakṛti; attachment to the guṇas is the cause of his births in good and evil wombs.
पुरुषः प्रकृतिस्थो हि भुङ्क्ते प्रकृतिजान् गुणान्कारणं गुणसङ्गोऽस्य सदसद्योनिजन्मसु
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
यत् पुरुषस्य सुखदुःखानां भोक्तृत्वं संसारित्वम् इति उक्तं तस्य तत् किंनिमित्तमिति उच्यते —
पुरुषः भोक्ता प्रकृतिस्थः प्रकृतौ अविद्यालक्षणायां कार्यकरणरूपेण परिणतायां स्थितः प्रकृतिस्थः, प्रकृतिमात्मत्वेन गतः इत्येतत् , हि यस्मात् , तस्मात् भुङ्क्ते उपलभते इत्यर्थः । प्रकृतिजान् प्रकृतितः जातान् सुखदुःखमोहाकाराभिव्यक्तान् गुणान् ‘सुखी, दुःखी, मूढः, पण्डितः अहम्’ इत्येवम् । सत्यामपि अविद्यायां सुखदुःखमोहेषु गुणेषु भुज्यमानेषु यः सङ्गः आत्मभावः संसारस्य सः प्रधानं कारणं जन्मनः, ‘सः यथाकामो भवति तत्क्रतुर्भवति’(बृ. उ. ४ । ४ । ५) इत्यादिश्रुतेः । तदेतत् आह — कारणं हेतुः गुणसङ्गः गुणेषु सङ्गः अस्य पुरुषस्य भोक्तुः सदसद्योनिजन्मसु, सत्यश्च असत्यश्च योनयः सदसद्योनयः तासु सदसद्योनिषु जन्मानि सदसद्योनिजन्मानि, तेषु सदसद्योनिजन्मसु विषयभूतेषु कारणं गुणसङ्गः । अथवा, सदसद्योनिजन्मसु अस्य संसारस्य कारणं गुणसङ्गः इति संसारपदमध्याहार्यम् । सद्योनयः देवादियोनयः ; असद्योनयः पश्वादियोनयः । सामर्थ्यात् सदसद्योनयः मनुष्ययोनयोऽपि अविरुद्धाः द्रष्टव्याः ॥
एतत् उक्तं भवति — प्रकृतिस्थत्वाख्या अविद्या, गुणेषु च सङ्गः कामः, संसारस्य कारणमिति । तच्च परिवर्जनाय उच्यते । अस्य च निवृत्तिकारणं ज्ञानवैराग्ये ससंन्यासे गीताशास्त्रे प्रसिद्धम् । तच्च ज्ञानं पुरस्तात् उपन्यस्तं क्षेत्रक्षेत्रज्ञविषयम् ‘यज्ज्ञात्वामृतमश्नुते’(भ. गी. १३ । १२) इति । उक्तं च अन्यापोहेन अतद्धर्माध्यारोपेण च ॥ २१ ॥
In plain words
It was said that the puruṣa is the one who undergoes pleasure and pain, the one caught in saṃsāra. And what causes that? That is what is spoken of now —
Puruṣa is called the experiencer 'seated in prakṛti'. What does that mean? Prakṛti here is ignorance itself, already shaped into a body and its instruments — and puruṣa is found sitting right inside that. Or, put differently, it means puruṣa has drifted into treating that very prakṛti as if it were himself. That's why he 'enjoys' — that is, actually feels — what happens in it. And what does he enjoy? The moods and states that prakṛti throws up: pleasure, pain, confusion, all dressed up as experiences he then claims as his own — 'I'm happy,' 'I'm miserable,' 'I'm lost,' 'I'm the clever one.' Even though it's all ignorance from the start, the real trouble isn't the pleasure or pain passing through — it's the clinging, the reflex of calling those passing states 'me'. That clinging is the true engine of birth, exactly as the scripture says: whatever a person wants, that's what he becomes bent on, and wherever his resolve goes, there he goes too. This is what's really meant by attachment to the guṇas — the puruṣa's own clinging is what plants him in one womb or another, good or bad. Good wombs mean the higher, godlike forms of life; bad wombs mean the animal and lower ones; and by the same logic, a human birth clearly belongs to this same in-between, mixed field, not somehow exempt from it.
So here is the whole point in one line: being 'situated in prakṛti' is just another name for ignorance, and clinging to the guṇas is just another name for desire — and together, these two are what actually cause saṃsāra. All of this is being spelled out for one reason only: so it can be let go of. And the way to let it go of is already fully worked out in this very text, in its teaching on knowledge and dispassion joined with renunciation. In fact that knowledge was already given earlier, under the heading of the field and its knower — remember the line, 'knowing this, one reaches the deathless' (Bhagavad Gītā 13.12). And notice how it was taught: first by clearing away everything that isn't the Self, and then by exposing and dropping every quality wrongly pinned onto the Self that never belonged to it.
Close to the Sanskrit
That which was stated — the puruṣa's being the experiencer of pleasures and pains, that is, his being bound to saṃsāra — with respect to that, what its cause is, is now said —
Puruṣa is the experiencer, 'situated in prakṛti': situated in prakṛti — characterized by ignorance, transformed into the form of effect-and-instrument — that is 'prakṛtistha'; or, having gone to take prakṛti as the Self — this is the meaning. Because — since — therefore, he 'enjoys', that is, apprehends: that is the meaning. He enjoys the guṇas born of prakṛti, arisen from prakṛti, manifested in the shape of pleasure, pain and delusion — thus: 'I am happy, I am unhappy, I am deluded, I am wise' — in this manner. Even though ignorance is present, while the guṇas of pleasure, pain and delusion are being undergone, the attachment which is self-identification with them — that is the principal cause of birth, as it is said: 'as is his desire, such becomes his resolve', and other such scriptures (Bṛhadāraṇyaka Up. 4.4.5). This very point he now states — the cause, the reason, is attachment to the guṇas: attachment to the guṇas, of this experiencing puruṣa, in births in good-and-bad wombs; good and bad wombs — 'sad-asad-yonayaḥ'; in those good-and-bad wombs, births — 'sad-asad-yoni-janmāni'; in those births in good-and-bad wombs, taken as the field — the cause is attachment to the guṇas. Or else: in births in good-and-bad wombs, of this saṃsāra, the cause is attachment to the guṇas — thus the word 'saṃsāra' is to be mentally supplied. Good wombs are the wombs of gods and the like; bad wombs are the wombs of animals and the like. By implication, human wombs too, being good-and-bad, are to be seen as not inconsistent with this division.
This amounts to saying: the state called 'situated-in-prakṛti' is ignorance; and attachment to the guṇas is desire; these are the cause of saṃsāra. And that is stated for the sake of its abandonment. And the cause of its cessation is well established, in the topic of knowledge-and-dispassion together with renunciation, within the Gītā treatise. And that knowledge was already presented earlier, having as its subject-matter kṣetra and kṣetrajña: 'having known which one attains immortality' (Bhagavad Gītā 13.12) — thus. And it was taught both by excluding what is other and by [negating] the superimposition of properties not belonging to it.