In Sāṅkhya-Yoga I became intrigued by the concept of
Puruṣa = svārtha, Prakṛti = parārtha.
i.e. the Spirit Self exists for itself, but Nature or Phenomena exist for another, i.e., the Puruṣa. (See Sāṅkhya-kārikā 17, 56, Yoga-sūtra 3.35 and their commentaries.)
The idea is that as conscious beings, we experience and process phenomena entirely as though we ourselves are the center of the universe. This has really nothing to do with selfishness in its ethical sense, though that is an outcome of this perfectly natural state. This is in fact one of Sāṅkhya philosophy's arguments for the existence of the spiritual self as separate and distinct from matter.
Any Vaishnava looking at Gītā 7.4-5 will recognize how the idea of the Puruṣottama adds a further dimension to this concept, but it is one that makes a huge difference.
bhūmir āpo'nalo vāyuḥ khaṁ mano buddhir eva ca
ahaṁkāra itīyaṁ me bhinnā prakṛtir aṣṭadhā
apareyam itas tv anyāṁ prakṛtiṁ viddhi me parām
jīva-bhūtāṁ mahā bāho yayedaṁ dhāryate jagat
Thus the individual soul or jiva is both Puruṣa (in relation to matter) and Prakṛti (in relation to the Puruṣottama).
Thus the Puruṣa is meant to employ Prakriti in the service of Puruṣottama.
Where is the Puruṣottama? How can we make the distinction between ātmā and Paramātmā ?
Bhagavad Gītā also takes up the Sāṅkhya concept of kṣetrajna and says there is one field-knower per field, but that the Supreme Field Knower, sometimes called "Cosmic Intelligence" by those who are reluctant to admit that Cosmic Intelligence is also attached to the Puruṣottama, pervades all fields.
But in terms of experience, the search for ātmā cannot be separated from the search for Paramātmā. One can only know the self truly in relation (parārtha) and not "in itself" (svārtha).
Sāṅkhya and Yoga say that the self cannot really be known in that way. As soon as one is self-reflective, one employs Buddhi, which is a material product. So because of this, they ultimately renounce even Buddhi to become Isolate (kevalin), where they live in splendid isolation, for themselves only.
The Vaishnava philosophy criticizes the concept of bliss that requires the renunciation of experience, especially the experience of love. Love is when svārtha and parārtha become one. That is when you know that both ātmā and Paramātmā have been realized.