Karika 19

tasmācca viparyāsāt siddhaṁ sākṣitvamasya puruṣasya। kaivalyaṁ mādhyasthyaṁ draṣṭr̥tvamakartr̥bhāvaśca ॥19॥

Based on that (multiplicity of beings) and deformations (of three qualities) we can establish the Purusha as a witness. It also establishes the Purusha's eligibility to liberation, intermediary nature, the observer, and not the doer feeling.