As an analytical system, Sankhya addresses observable facts and develops explanations to encompass those observations. This led Sankhya to develop, first of all, a dualistic system of Purusha and Prakriti. Purusha, the unmoving, non-acting witness consciousness which reflects the action of mechanical Nature.
It was observed that Prakriti follows a consistent pattern everywhere, and is one encompassing system. It was also noted, however, that the individual experience varied, and this led Sankhya to posit not only the dualism of Purusha and Prakriti, but also a pluralistic system of multiple Purushas.
Sri Aurobindo describes the factors that led Sankhya to adopt this position: “First, actually, we find that there are many conscious beings in the world and each regards the same world in his own way and has his independent experience of its subjective and objective things, his separate dealings with the same perceptive and reactive processes. If there were only one Purusha, there would not be this central independence and separativeness, but all would see the world in an identical fashion and with a common subjectivity and objectivity. Because Prakriti is one, all witness the same world; because her principles are everywhere the same, the general principles which constitute internal and external experience are the same for all; but the infinite difference of view and outlook and attitude, action and experience and escape from experience,–a difference not of the natural operations which are the same but of the witnessing consciousness,–are utterly inexplicable except on the supposition that there is a multiplicity of witnesses, many Purushas.”
Sri Aurobindo points out the logical necessity that eventually was one of the major factors that led to Sankhya’s determination: “The cosmos and its process can be explained by the commerce of one Prakriti with one Purusha, but not the multiplicity of conscious beings in the cosmos.”
Sri Aurobindo, Essays on the Gita, First Series, Chapter 8, Sankhya and Yoga, pp. 68-69,