Karika 20

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

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.