Chapter 8, Verse 22
Part of 8: Akṣhar Brahma Yog — अक्षरब्रह्मयोगपुरुषः स परः पार्थ भक्त्या लभ्यस्त्वनन्यया। यस्यान्तःस्थानि भूतानि येन सर्वमिदं ततम्॥
puruṣhaḥ sa paraḥ pārtha bhaktyā labhyas tvananyayā yasyāntaḥ-sthāni bhūtāni yena sarvam idaṁ tatam
That highest Purusha, O Arjuna, is attainable by unswerving devotion to Him alone, within Whom all beings dwell and by Whom all this is pervaded.
All the beings (effects) dwell within the Purusha (the Supreme Person, the cause) because every effect rests within its cause. Just as the effect, pot, rests within its cause, the clay, so also all beings and the worlds rest within their cause, the Purusha. Therefore the whole world is pervaded by the Purusha. Sri Sankara explains exclusive devotion as Jnana or knowledge of the Self. Purusha is so called because everything is filled by It (derived from the Sanskrit root pur which means to fill) or because It rests in the body of all (derived from the Sanskrit root pur). None is higher than It and so It is the Supreme Person. (Cf. IX.4XI.38XV.6and7)