Samkhya Karika 19

तस्माच्च विपर्यासात् सिद्धं साक्षित्वमस्य पुरुषस्य ।
कैवल्यं माध्यस्थ्यं द्रष्टृत्वमकर्तृभावश्च ॥ १९ ॥

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

Tasmāt=from that; ca=and; viparyāsāt=from contrast; siddhaṁ=is established; sākṣitvam=being a witness; asya=its; puruṣasya=of the Purusha; kaivalyaṁ=isolation; mādhyasthyaṁ=neutrality; draṣṭṛtvam=being an observer, perceiver; akartṛ=non-doer; bhāvaś=attitude; ca=and

And, from that contrast it is established that the Purusha is the pure witness. He is ever free, neutral, a spectator and a non-doer.

According to commentators, the contrast, mentioned in this karika actually refers back to karika #11 – “The manifest is composed of the three gunas (sattva, rajas and tamas), is indiscriminative, objective, common, without consciousness, and productive. Prakriti (Pradhana) is also like this. Purusha, though similar, is yet the reverse of all these.” The current karika states that due to the contrasting attributes of purusha and prakriti, several other attributes of purusha, listed in this karika, can be established.

From reading the commentaries, it is not very clear how Purusha, just by being devoid of the three gunas, can be identified with the attributes listed here. I am giving here an excerpt from the commentary by Gaudapada (translated by Manikar):

“Thus, Sattva, Rajas and Tamas being the agents, it follows that the Spirit is a passive witness, that Spirit which is the subject of plurality. These Attributes alone are the agents and act; the witness neither acts nor desists from acting. Again, detachment, isolation is an attribute of the Spirit; Kaivalya is the property of being sole, having detachment or isolation or distinctness from others; that is, it, the Spirit, is distinct from the three Attributes, and from that which is possessed of the three Attributes. Indifference, being a bystander is the condition of a middle man, the Spirit is indifferent like a wandering mendicant. Just as an ascetic is indifferent while the villagers are engaged in tilling and agriculture, so the Spirit is also indifferent while the Attributes are active. Therefore, the Spirit is endowed with perception and inactivity. Since the Spirit is a bystander and indifferent, therefore it is a spectator and therefore also the non-agent of those actions.”