पुरुषः प्रकृतिस्थो हि भुङ्क्ते प्रकृतिजान्गुणान्। कारणं गुणसङ्गोऽस्य सदसद्योनिजन्मसु ॥

puruṣhaḥ prakṛiti-stho hi bhuṅkte prakṛiti-jān guṇān kāraṇaṁ guṇa-saṅgo ’sya sad-asad-yoni-janmasu

Spirit seated in Matter useth the qualities born of Matter; attachment to the qualities is the cause of his births in good and evil wombs.

In plain words: The Spirit lives in the body and experiences the qualities that come from nature. Because it gets attached to those qualities, it is born again and again, sometimes in good lives and sometimes in bad ones.