Karika 20

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

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.