It is imagined in the Mahabharata that the all-pervasive entity (puruṣa) resides in each body bearing the size of a thumb. Prādesamatra Purusa having taken shelter in subtle body transmigrates. Highly elevated yogis only can see the self (Pratyagatma) —

Aṅguṣṭhamātraḥ puruṣo’ntarātmā
Liṅgasya yogena sa yāti nityam.
Tamīśamīḍymanukalpamādyaṃ
Paśyanti mūḍā na virajamānam.
Yoginastaṃ prapaśyanti bhagavantaṃ sanātanam.

Although the hermit sages can see the phantasm with their yogic sight, as this manifestation of the animal self or soul is too minuscule to discern visually, it is impossible for the laymen to witness the phantasmal manifestation. It is for this reason that in Kaṭhaśruti (Kathsruti) this manifestation of the soul (puruṣa; purusha) has been described as ‘measuring only a finger’ (aṅguṣṭha-mātra; angushtha-matra). Describing the measurement of the phantasm, this particular verse of Kaṭhaśruti continues that this thumb-measured, subtle soul resides in the physical body as the ruler of the past, the present and the future and knowing Him as the ruler of the past and the future, one does not shrink (from Him). Kaṭhaśruti, considering the soul to be the ultimate abode of human ego, intelligence and all other faculties, says —

Aṅguṣṭhamātraḥ puruṣo madhya ātmani tiṣṭhati.
Īśāno bhūtabhavyasya na tato vijugupsate etadvaitat.

The verse next to this one in Kaṭhopaniṣada (Kathopanishada) mentions this aṅguṣṭha-mātra puruṣa again and compares him with an incandescent, smokeless flame. He exists today and will exist tomorrow. (Katha 2.1.13). In the commentary of the said mantras Śaṅkarācārya (Sankaracharya) equalizes the thumb-sized puruṣa (purusha; the Soul) with subtle body. Non-dualist Śaṅkara, the founder of the advaita vedānta (advaita vedanta) school of philosophy, also explains it as associate (upādhi; upadhi) of antaḥkaraṇa (antahkarana; the inner Self). That which makes a thing appear different from what it is. Being conditioned by the internal organ existing in the space within the lotus of the heart, just like space existing in a section of a bamboo, the thumb-sized puruṣa dwells in a material body. Śaṅkarācārya says that the soul which is said to be of the size of a thumb is in reality Brahman. 

Although sāṃkhya (samkhya) cosmology denies supreme existence but in admitting individuals’ enjoyment and liberation it acknowledges some instruments like the sense organs, pañcatanmatra (panchatanmatra; five subtle elements) and pañcabhuta (panchabhuta five gross elements). It is a debatable point among scholars for long whether these organic instruments and the karmic results might be carried on as residual burden during transmigration of the soul. The movement of organs inherent in the soul during transmigration from one body to another has been poetically described in the Manu-Bṛhaspati dialogue in Mahābhārata. In this episode it is said that (as can be found in Haridasa Siddhantabagish’s commentary of Mahābhārata) just as in a dream a person may consider his own body, dropped on the ground, as that of another, similarly the animal soul or self (jīvātman; jivatman) along with five organs of sense, five vital airs, wind and intellect transmigrates from one body to another.

Yathātmano’ṅgaṃ patitaṃ prithīvyāṃ

Svapnāntare paśyati cātmano’nyat.

Śrotrādiyuktaḥ sumanaḥ sabuddhi

Liṅgāttathā gacchati liṅgamanyat. 

Manusaṃhitā (Manusamhita) also is pertinent in this regard. During complete dissolution of the universe (mahāpralaya; mahapralaya) individual soul having fallen into nescience (ajñāna; ajnyana) long remains with it organs, does not perform its own functions and departs from its material frame.

tamo’yaṃ tu samaśritya ciraṃ tiṣṭhati sendriyaḥ

na ca svayaṃ kurute karma tadotkrāmati mūtitaḥ. 

When having become elemental, i.e. encased in the astral body (composed of the eight constituents), it enters the mobile (i.e. animal), or immobile (i.e. vegetable) seed, it assumes a gross body. This essence has been hailed anumātrika vīja (anumatrika vija) of Manusaṃhitā and the subtle body (or soul) spoken about in Mahābhārata and Purāṇa are identical.

Departure of a soul is described as transmigration [utkramaṇa (utkramana) or utkrānti (utkranti)] in Bṛhadāraṇyaka Upaniṣada (Brihadaranyaka Upanishada). In the Manu text the term utkrāmati has also been used. Medhātithi (Medhatithi), the commentator of Manusamhita, raises a question while writing the commentary of Manusaṃhitā in the following way. If atman is all-pervading like sky, how can it be transmigrated? Only limited or confined object of knowledge may go to a different place. Therefore, transmigration of soul is theoretically absurd. And Medhātithi himself provides an answer. He says that the present body is obtained through the karmic results of the past life actions. At one point the soul dissociates itself from the (one) current animal body and its special relation with body is broken; this departure of the soul is called utkrānti or transmigration. Medhātithi does not equalize the movement of a material element with transmigration of soul. In supporting of his statement, Medhātithi followed the conventional philosophical views of the earlier age. In the interim period of the casting off present body and accepting to future body, a separate subtle body is formed. This subtle body is called ātivāhika śarīra (ativahika sharira) or antarābhava śarīra (antarabhava sharira). This ātivāhika śarīra, unlike the mortal body, is beyond all earthly sensations. This celestial body only transmigrates. It cannot be destroyed by any means. Even five gross elements (pañca mahābhūta; pancha mahabhuta) can never create any kind of obstruction to its arrival and departure.

Both Medhātithi and Kullūkabhaṭṭa (Kullukabhatta), while explaining the formation or constitution of this subtle body or ātivāhika śarīra or antarābhava śarīra, mention the term puryaṣṭaka (puryashtaka) from Brhamapurāṇa. Puryaṣṭaka is formed by eight (aṣṭaka; ashtaka) constituents (purī; puri). In the boundless cosmos, between two births this subtle body becomes the place of dwelling for each soul for the interim period, their status reflecting the merits and demerits of the performed acts of the previous birth. According to Purāṇas, the soul is attached to the subtle body in the form of puryastaka and this period is known as prāravdha (praravdha; fruit yielding karmans), which only remains. The results of prāravdha have to be endured by the jīva (jiva; soul) because their purification is contrary to the Lord’s will. Bondage and liberation both are on the jīva. If jīva is tied by puryaṣṭaka, then comes bondage; if jīva is free from puryaṣṭaka then liberation beckons.

According to Medhātithi, the subtle body or liṅga śarīra is formed by eight elements: pañcaprāṇa (panchaprana), [namely prāṇa (prana), apāna (apana), samāna (samana), udāna (udana) and vyāna (vyana)], the collection of five organs of sense, the collection of five organs of action and the mind. However, Kullūkabhaṭṭa, the renowned annotator of Manusaṃhitā, does not support Medhātithi in this regard. He acknowledges that puryaṣṭaka or liṅgaśarīra is constituted by eight elements, but differs in the ingredients. Citing his predecessor Sadānanda (Sadananda) or Sananda, he observes that puryaṣṭaka consists of the united form of pañcabhūta, the united form of pañcendriya (panchendriya; the five senses), the united form of pañcavāyu (panchavayu; same as pañcaprāṇa), mind, intellect, desire, action and avidyā (avidya; nescience). Allegorically subtle body or liṅga śarīra or antarābhava śarīra is called aṅguṣṭha.