Now in order to understand this one sentence, we have to stop here for quite some time, because this is one of the most important statements in the Gita and therefore I don’t want to hurry up in order to move forward. The reason why it is important to halt here is that it deals with the totality of the understanding of the universe as you find in the development of human thought and particularly in the Indian thought. In India almost every religious thought speaks of Purusha and Prakriti. I don’t think anybody in this country does not use these two words at least once or twice in his life: Purusha and Prakriti. And there is so much of confusion on this subject, even in the Indian philosophical thought, there is considerable confusion. It is for that reason that we have to understand in a greater clarity this very important statement.
To start with we take up what is called the concept of Purusha and Prakriti in Sankhya. There is a philosophical system which is called the Sankhyan philosophy. The word Sankhya also means ‘the philosophy which deals with Sankhya’, which deals with numbers. It is because this philosophy enumerates the most essential elements of the world and comes to the conclusion that there are 25 of them. Since it gives a detail of this enumeration and arrives at 25, gives a number, (Sankhya), therefore also this is called ‘Sankhya’.
The first among them is ‘Purusha’; it is one, one principle. The rest 24 all belong to Prakriti. Prakriti gives rise to mahat (or Buddhi), which gives rise to ahaṁbhāva (egoism), which is three fold: Sattwic ahaṁbhāva , Rajasic ahaṁbhāva , Tamasic ahaṁbhāva , because Prakriti itself is threefold (Sattwic, Rajasic, Tamasic). And everywhere in the world these three are pervading, universal pervasion.
From them arise 6 senses: manas first which is the real 6th sense, and the real sense and 5 senses of knowledge and 5 senses of action. From there arise the essences, which are called tanmātrā(s), of five pañca mahabhūta(s) of the five great elements: ether, air, fire, water, and earth. They all come to 24 and Purusha is 25th.
Now, according to Sankhya philosophy, this knowledge is necessary because through this knowledge one knows what is bondage, and what is liberation (what is mokṣa). The Purusha according to this philosophy is by nature inactive: it is quiet, but luminous, (full of consciousness). There is not one Purusha but many Purushas: each one of us is a Purusha. The word Purusha is puri vasati yaḥ saha puruṣaḥ, that which lives in town, (the body is a town), that which lives in the body is Purusha.
So, everyone is Purusha; every one of us, basically, is Purusha, that is to say ‘body is not Purusha’, body is the ‘town’ in which we live. The ‘one’ who lives dehi, not deha, dehi one who lives in the deha is Purusha.
Question: puruṣa, this would be different from kṣetrajña?
kṣetrajña is Purusha but more than that.
Purusha in the Sankhya is not Purushottama. In the Bhagavad Gita, the word Purusha is used also in the sense of Purushottama, therefore in the Bhagavad Gita kṣetrajña would certainly be Purusha, but Purushottama is also kṣetrajña: one who knows the field, Purushottama Himself, the supreme Lord is the kṣetrajña.
In Sankhya there is no concept of Purushottama at all, there is only the concept of Purusha.
In the Bhagavad Gita, the many Purushas are not original. The multiplicity of Purushas is a result of Purushottama. Therefore Purusha basically is only one according to Bhagavad Gita.
Sankhya is called pluralistic philosophy, according to it, ultimate Reality is multiple, many Purushas, so it is called a philosophy of pluralism. Bhagavad Gita maintains that ultimate Reality is only one, therefore Gita’s philosophy is called ‘monism’, it is therefore ‘advaita’ (not two), Adwaita it is one without duality, ultimate Reality is one without the second: there are not two ultimate realities.
So, according to Sankhya…(I am just now discussing only Sankhya, but since your question was very important, I expounded immediately the Bhagavad Gita’s position vis à vis the position of the Sankhya)…the Bhagavad Gita very often uses the word Sankhya, through out the book we have seen the word Sankhya, and it seems as if Sri Krishna describes this Sankhya’s philosophy which I am just now describing, but that is not true, the word Sankhya wherever used by the Bhagavad Gita is the “Vedantic Sankhya”, it is not the Sankhya which is known ultimately by the name of “Sankhya philosophy” in Indian philosophy.