According to the Bhagavad Gita, the word Sankhya is used to indicate all that I have described so far plus the idea that Purusha is not multiple but one. According to Vedantic Sankhya which is the Gita’s Sankhya, Purusha is one, (not many), therefore it is called ‘monistic’, not ‘dualistic’, but monistic. Bhagavad Gita’s Sankhya is monistic, Vedantic Sankhya is monistic, there is only one Reality. Even the so called Prakriti which is regarded separate from Purusha in the Sankhya is according to the Gita a power of Purusha Himself: it is not something different from Purusha. Prakriti is itself power of Purusha, and in regard to Prakriti, Purusha has four functions. In the case of Sankhya, as we understand normally, Purusha has only two functions: one is that it is draṣṭā, it is a witness; and it is bhoktā, it enjoys; it is also anumantā, it is also the one who gives sanction, but very passively. In the case of Bhagavad Gita’s Sankhya, which is Vedantic Sankhya, the Purusha is first of all Maheshwara (maheśvara), He is the supreme Lord: so, Prakriti is under the control and command of Purusha. He is bhartā, He is not only Maheshwara, but he is also bhartā; bhartā is the one who actually fills Prakriti with all His energy. Prakriti would be nothing if this bhartā and this Purusha were not there. It is Purusha who fills; it is Purusha who husbands Prakriti. He is also bhoktā, also anumantā, He also gives sanction but not like Purusha of Sankhya, where Purusha gives sanction passively. In the Bhagavad Gita’s view, Purusha is an active sanctioner. And He is also draṣṭā, He also witnesses.
These are some of the great statements of the Bhagavad Gita: what is the nature of Purusha? In one of the chapters, you will come across this very description, very clearly: draṣṭā, anumantā, bhartā, bhoktā, maheśvara. In other words, Purusha according to Bhagavad Gita is Ishwara (īśvara) also. Purusha is also Brahman. According to Sankhya as we understand it today, Purusha is not Brahman, Purusha is not Ishwara: in the Bhagavad Gita, the Purusha is ‘at once’ Brahman, Purusha, Ishwara.
And the energy, what is called Prakriti in the Sankhya, ordinary Sankhya, Sankhya as we understand it, is again three fold: it is the power of the Supreme, therefore it is the Shakti: Prakriti is Shakti. Prakriti is Maya (māyā), in the sense that it is that which measures the ‘Immeasurable’: not in the sense of illusion, but in the original sense, the word Maya means: ‘that which measures’. The original word is mā, yā māti sā māyā, that which measures, the Reality is above all measure, is immeasurable; the Immeasurable is being measured because all the forms that we see are all measurements. The forms are brought out from the Immeasurable, and that is the function of Maya. In so far as the energy of the Lord, measures out from the immeasurable, she is also the Maya. She is also Prakriti, in the sense that she is an obedient servant of the Purusha, of the Brahman, of the Ishwara.