Brihadaranyaka Upanishad 1.4.7

1.4.7

taddhedaṃ tarhyavyākṛtamāsīttannāmarūpābhyāmeva vyākriyatāsau nāmāyamidam̐ rūpa iti tadidamapyetarhi nāmarūpābhyāmeva vyākriyate'saunāmāyamidam̐rūpa iti sa eṣa iha praviṣṭaḥ ā nakhāgrebhyo yathā kṣuraḥ kṣuradhāne'vahitaḥ syādviśvaṃbharo vā viśvaṃbharakulāye taṃ na paśyanti | akṛtsno hi saḥ prāṇanneva prāṇo nāma bhavati | vadanvāk paśyam̐ścakṣuḥ śṛṇvanśrotraṃ manvāno manastānyasyaitāni karmanāmānyeva | sa yo'ta ekaikamupāste na sa vedākṛtsno hyeṣo'ta ekaikena bhavatyātmetyevopāsītātra hyete sarva ekaṃ bhavanti | tadetatpadanīyamasya sarvasya yadayamātmānena hyetatsarvaṃ veda yathā ha vai padenānuvindedevaṃ kīrtim̐ ślokaṃ vindate ya evaṃ veda

This (universe) was then undifferentiated. It differentiated into name and form—it was called such and such, and was of such and such form. So to this day it is differentiated into name and form—it is called such and such, and is of such and such form. This Self has entered into these bodies up to the tip of the nails—as a razor may be put in its case, or as fire, which sustains the world, may be in its source. People do not see It, for (viewed in Its aspects) It is incomplete. When It does the function of living, It is called the vital force; when it speaks, the organ of speech; when It sees, the eye; when It hears, the ear; and when It thinks, the mind. These are merely Its names according to functions. He who meditates upon each of this totality of aspects, does not know, for It is incomplete, (being divided) from this totality by possessing a single characteristic. The Self alone is to be meditated upon, for all these are unified in It. Of all these, this Self alone should be realised, for one knows all these through It, just as one may get (an animal) through its footprints. He who knows It as such obtains fame and association (with his relatives).