From Samkhya's Metaphysics to Yoga's Discipline

Samkhya is counted among the oldest of classical India's six systems of philosophy, and its account of purusha and prakriti supplied the metaphysical foundation an entirely different school later built a practical discipline on top of. The Yoga system associated with Patanjali adopts Samkhya's dualism of consciousness and nature almost without alteration, accepting the same twenty five tattvas and the same picture of prakriti unfolding into mind and matter while purusha stands apart from it as pure witness. Where Yoga departs from its source is in adding a twenty sixth element missing from classical Samkhya entirely: Ishvara, a particular, distinguished form of purusha untouched by affliction, action or its results, who serves in the Yoga system as an object of devotion and a support for meditative concentration rather than as a creator god responsible for the universe's existence. Classical Samkhya itself has no need of such a figure and no settled position on whether one exists, since liberation in its own system comes purely through discriminating knowledge, the direct recognition that one is purusha and not prakriti, with no divine assistance required to reach it. That difference marks the clearest line between the two schools even as Yoga otherwise inherits Samkhya's picture of the cosmos nearly whole, which is why the two are so often described as a paired philosophy rather than as two unrelated systems that happen to share some vocabulary.