Sankhya posited multiple Purushas to solve several issues; the Gita, however, does not accept Sankhya’s solution in this regard and, while it accepts most of the concepts of Sankhya as valuable tools for acquiring and organising knowledge, it provides an alternative view on this major issue.
Sri Aurobindo describes the Gita’s adaptation: “The first important new element we find is in the conception of Purusha itself. Prakriti conducts her activities for the pleasure of Purusha; but how is that pleasure determined? In the strict Sankhya analysis it can only be by a passive consent of the silent Witness. Passively the Witness consents to the action of the intelligent will and the ego-sense, passively he consents to the recoil of that will from the ego-sense. He is Witness, source of the consent, by reflection upholder of the work of Nature….but nothing more.”
“But the Purusha of the Gita is also the Lord of Nature; he is Ishwara. If the act of intelligence of the Will is the act of Prakriti, the source and light of the intelligence are actively contributed by the Purusha; he is not only the Witness, but the Lord and Knower, master of knowledge and will….”
“He is the supreme cause of action of Prakriti, the supreme cause of its withdrawal from action. In the Sankhya analysis Purusha and Prakriti in their dualism are the cause of the cosmos; in this synthetic Sankhya Purusha by his Prakriti is the cause of the cosmos.”
The dualism of the Sankhya eventually breaks down when we begin to consider “first causes”. The Gita’s approach provides a solution that addresses both the issues that Sankhya confronted as well as the “first cause” issue, and is thus, an evolutionary step forward in our understanding of existence. This provides the further developments that the Gita proposes as the teaching develops.
Sri Aurobindo, Essays on the Gita, First Series, Chapter 8, Sankhya and Yoga, pp. 70-71,