Karika 20

तस्मात्तत्संयोगादचेतनं चेतनावदिव लिङ्गम्। गुणकर्तृत्वे च तथा कर्तेव भवत्युदासीनः ॥२०॥

tasmāttatsaṁyōgādacētanaṁ cētanāvadiva liṅgam। guṇakartr̥tvē ca tathā kartēva bhavatyudāsīnaḥ ॥20॥

[तस्मात् तत् संयोगात् अचेतनं चेतनावत् इव लिङ्गम्। गुण कर्तृत्वे च तथा कर्त इव भवति उदासीनः॥
tasmāt = therefore; tat = of that; saṁyōgāt = by union; acētanaṁ = non living; cētanāvat = with the living; iva = only; liṅgam = physical form; guṇa = qualities; kartr̥tvē = through instrumentality; ca = and; tathā karta = thus; iva = is known; bhavati = becomes; udāsīnaḥ = indifferent / lethargic;]

Therefore, by its union, the body without spirit gets recognized as living and with spirit. Because of Purusha's indifferent and lethargic nature, he gets recognized as a doer, though qualities are the instruments.

It is critical to recognize the indifferent and lethargic nature of purusha as a defect and the cause of it recognizing itself as an instrument.