Alongside Prakṛti, Sāṅkhya-Yoga theorizes that it has a constant counterpart in the form of Puruṣa. Thus, existence is a kind of duality composed of these two principles—material (Prakṛti) and spirit (Puruṣa).
Prakṛti (the physical universe) is said to evolve for the sake of Puruṣa (the spiritual consciousness that underlies it).
By analogy, the evolution of the universe towards a state of spiritual perfection is like a man evolving or improving himself towards achieving an ideal.
Puruṣa is the spirit which is inherent within nature, and thereby seeks to escape from the physical state of nature into a spiritual state of existence.