Per Samkhya Karika (18) Purushas are many because birth, death and actions are specific to individuals, because many simultaneous tendencies or modifications are impossible in one individual and because modifications arising from the three gunas are various.
Birth and death are corporeal properties. The body suffers bhoga (i.e. experiences pleasure and pain), the bhokta (one who experiences) of which must be a single entity for bhoga arising from one particular body. Since there are many bodies or experiential beings, each must have a distinct bhokta. Therefore bhokta Purushas are many.
The Karanas or the instruments effect perception and action. They are able to perform harmoniously by virtue of an individual Seer above them. As there are many collections of instruments in many corpora, their Seers must also be many.
Conation is effort of mind and is a temporal state. The Seer that is individual and ‘One’ must ‘see’ one such state at a time. But since there exist many being with many states at the same time, it must be admitted that ‘there are many Seers’. In fact ‘seeing’ one state at a time by the same Seer signifies His indivisibility and unity. If we say the He is the Seer of many states at a time, we are referring, in reality to many Seers. The ‘one’ Seer of the previous statement is a collective ‘one’, like ‘one’ forest is a collection of many trees.
The indiscrete state (Pradhana or Prakrti) which is made up of the three gunas is one. Purusa is its opposite and hence must be many. Had there been only one Seer and one prakrti, there would have been only one living being. The origins of many objects must be many. Prakrti is one. Therefore many causes are needed for its multifarious modifications. The immutable cause of modification of prakrti into buddhi (pure-I sense) etc. is Purusa. Therefore many Purusas are needed to lead to many buddhis. If we say that the cause of many buddhis is ‘one Purusa‘, this ‘one’ Purusa will be a collective ‘one’ and not an indivisible ‘one’.
There are many Purusas because of these reasons. But there is another reason for it viz. for understanding moksa (liberation). When a living being discards his illusion of non-self and remains in oneself, he is liberated. One does not feel then that ‘I have become the soul of all living beings’ since then all dualisms such as ‘all’, ‘living being’ etc. must be discarded.
The pluralism of Purusa is the tenet of almost all Indian philosophical systems e.g. Samkhya, Nyaya, Purva Mimamsa, Ramanuja’s philosophy etc. Only Vedantists (Mayavadins) oppose it. They say that Purusa or Atman is one. They can proffer no argument supporting it, saying only that ‘It is in the scriptures’. Certain statements of the Upanishads do appear, at first glance, to support a monistic view, but they really mean something else. An example is the oft quoted saying of Katha Upanishad (2.2.9) – ‘As one fire entering the world appears as many fires, so the Soul appears as many because of its presence in each being; He is also present externally.’
The Atman of this Sruti is not the nirguna, immutable, Cit-only Brahman as He never mutates. The Atman here is Prajapati Hiranyagarbha, on whose I-sense this world exists. It is also well known that ‘Atman‘ is used to mean Hiranyagarbha also, as for example in another Sruti (Mundaka Upanishad 2.2.7)
Divye brahmapure hyesa vyomni atma prathistitah.
The nirguna Cit-only Atman can never be established in svarga (heaven), but everything including heaven and hell is established in His awareness. Therefore this ‘Atman‘ established in the space of brahmapura is a particular Purusa who exists in brahmaloka; He is not the nirguna Cit-only Purusa. The cofusion arises because the Upanishads are not a single philosophical text. Being ancient, their language is often loose and they were also written over a long span of many years. Therefore it is a mistake of the Vedantists to consider all of them as having the same connotation. They have to stretch the meaning of the text a little too much to suit their logic. Samkhyas possibly do the same (but they do not violate fundamental rules of logic in the process).
Let us now examine the theory of the Vedantists regarding the oneness of Purusa. When they encounter such a statement of the Upanishad as above, they say that there is one Atman in all living beings, who does not suffer birth or death, which pertains only to attributes such as body etc. The attributes may vary but not the One whose attributes they are. As one sky looks distinct and different from different houses, enclosures etc., so does one Atman appear as many being reflected in many buddhis. As one sun appears as many being reflected in the water in many containers, so does one Atman appear as many being reflected in many buddhis.
In reply, one may ask why there should be one Atman. The Vedantists are silent on this issue. They first assume it to be so and then supply a few similes (not examples) in conformity with it. It is clear that the Atman is without birth and death, which are properties of bodies etc. Samkhya does not make such childish statements. On differences of characteristics, Samkhya-sutra (1.50) says – ‘Plurality is added to one by difference in characteristics, as the sky appears many due to (its reflections in) many pots.’ Samkhya-sutra (1.51) elaborates further and says – ‘In such cases, only characteristics vary. That on which they apply does not vary.’
Such a premise is surely true, but if from this one deduces that Atman, by attribution of many characteristics on Him, appears as many, that would be incorrect. This is because as the next Samkhya-sutra (1.52) shows – ‘The Knower that is indivisible One i.e. who appears to mutate as one unit, cannot be ascribed with different characteristics at the same time. At any time there cannot be ascribed to any object different characteristics by the same Knower. In other words, the same Seer cannot have different (either consonant or opposite) perceptions at the same time; such a thing is not even conceivable. (According to Vedantists, Brahma is the only Knower and must therefore see or perceive that ‘I am liberated’ and ‘I am bound’ at the same time. But they are unable to say clearly who it is who perceives.)
If you posit the example of space (sky that we see is, in reality, an imaginary entity) that would not be proper. Space has spatial extent, which appears different to different observers when it pertains to different physical extents. The same flaw applies to the simile of the sun. Thus those who seek to understand oneness of Atman make a mistake at the very beginning – viz. they imagine Atman to be a material entity with very large (even infinite) spatial extent. The indivisible Seer without spatial extent can only have (by Himself) the perception of one thing at a time. Different perceptions at the same time must entail the perceivers to be many spatial entities. It is ridiculous to consider the Atman as having spatial extent. Therefore it cannot be established that the Atman is one in number. Had space been an indivisible Seer, and could prove that it has simultaneously many perceptions, then only would the simile be true… (from Samkhya Across the Millenniums by Swami Hariharananda Aranya)