Patanjali has defined Yoga in the first 3 sutras.
I.-2 (Yoga is achieved through calming the mind by restraining it from its natural tendency of taking various forms.)
Later, he describes how perception creates forms because of guṇa in the perceived object, and how all this is handiwork of Prakṛtī.
I-3 (When this is accomplished, the seer knows what he or she really is.)
This ultimate, real identity has not been explicitly mentioned, but pointers are provided in sutra I-16 and IV-34.
I-16 (When non-attachment becomes a way of life, one is liberated from the qualities of guṇa and possesses knowledge of the spiritual being that one is.)
Thus it is implied that a seeker?s quest for knowing the real identity travels an evolutionary path culminating in a realization that he or she is really a ?spiritual being?.
IV-34 (The pure spiritual consciousness withdraws into One.)
If one is in doubt about what kind of spiritual being, Patanjali describes it as ?a state of isolated unity? which becomes possible, when the three guṇas (qualities of Prakṛtī) no longer exercise any hold, as Purūṣa is isolated from it. This then is a resolution of a conflicting duality (the primary pair of opposites, Purūṣa and Prakṛtī) into the ultimate non-duality, Purūṣa. Thus, a seeker reaches the ultimate goal in Yoga, in realization that he/she is Purūṣa.
Use of the word ?pratiprasavā? is significant here. It literally means a process exactly reverse of ?giving birth?. Patanjali is referring to the original event of birth of matter (Prakṛtī) from the potent nothingness (Purūṣa) and is saying that Yoga is a reverse process where Prakṛtī merges back into Purūṣa.
Patanjali also elaborates the process of Yoga by telling us why duality, pairs of opposite, exists in III-35 and how that can be overcome in III-49.
III.-35 (Experience of the pairs of opposites comes from the inability to distinguish between the personal self and Purūṣa, the Spirit, a confusion that is clarified by Saṁyama on Self.)
III-49 (One who can discriminate between the Soul and Spirit achieves supremacy over all conditions and becomes omniscient.)
Patanjali doesn?t forget to highlight the importance of purification through the eight-fold practice of Yoga in achieving the goal, in sutra III-55.
III-55 (When all the bodies (physical, astral and causal) and the soul have reached the condition of equal purity, then true one-ness is achieved and liberation results.)