The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
29. But it is designated thus (i.e. as atomic), on account of its having for its essence the qualities of that (i.e. the buddhi); as in the case of the intelligent Self (i.e. Brahman).
The word 'but' is meant to set aside the opinion maintained hitherto.--The soul is not of atomic size, since scripture does not declare it to have had an origin. On the contrary, as scripture speaks of the highest Brahman entering into the elements and teaches that it is their Self, the soul is nothing else but the highest Brahman. And if the soul is the highest Brahman, it must be of the same extent as Brahman. Now scripture states Brahman to be all-pervading. Therefore the soul also is all-pervading.--On that view all the statements about the all-pervadingness of the soul made in Sruti and Smriti are justified, so, for instance, the passage, 'He is that great unborn Self who consists of knowledge, is surrounded by the prânas &c.' (Bri. Up. IV, 4, 22). Nor again could the soul, if it were of atomic size, experience sensations extending over the whole body. If it be said that that is possible owing to the soul's connexion
with the sense of touch (the skin), we deny that assertion. For from that it would follow that, when we tread on a thorn, the sensation extends over the whole body, since the connexion of the thorn and the skin abides in the entire skin, and the skin extends over the whole body. While as a matter of fact, when treading on a thorn we experience a sensation in the sole of the foot only.--Nor again is it possible that a quality of an atom should diffuse itself beyond the atom. For qualities occupy the same place with the substances of which they are qualities, and a quality not abiding in its substance would no longer be a quality. Concerning the light emitted from a lamp we have already shown that it is, not a quality, but rather a different kind of substance. Hence odour also, being avowedly a quality, can exist in so far only as it inheres in its substance; otherwise it would cease to be odour. Thus the reverend Dvaipâyana also says, 'Having perceived odour in water some unthinking people ascribe it to the latter; but know that it is in the earth only, and (merely) passes over into air and water.' If the intelligence of the soul pervades the whole body, the soul cannot be atomic; for intelligence constitutes the soul's proper nature, just as heat and light constitute that of fire. A separation of the two as quality and that which is qualified does not exist. Now it has already been shown (II, 2, 34) that the soul is not of the same size as the body; the only remaining alternative therefore is that it is all-pervading (infinite). But why then, our opponent asks, is the soul designated (in some scriptural passages) as being of atomic size, &c.?--It is designated as such 'on account of being of the nature of the essence of that (i.e. the buddhi).'--The Self is here said to be of the nature of the essence of the mind's (buddhi) qualities, because those qualities, such as desire, aversion, pleasure, pain and so on, constitute the essence, i.e. the principal characteristics of the Self as long as it is implicated in transmigratory existence. Apart from the qualities of the mind the mere Self does not exist in the samsâra state; for the latter, owing to which the Self appears as an agent and enjoyer, is altogether due to the circumstance of
the qualities of the buddhi and the other limiting adjuncts being wrongly superimposed upon the Self. That the non-transmigrating eternally free Self which neither acts nor enjoys is declared to be of the same size as the buddhi, is thus due only to its having the qualities of the buddhi for its essence (viz. as long as it is in fictitious connexion with the buddhi). Moreover we have the scriptural passage, 'That living soul is to be known as part of the hundredth part of the point of a hair, divided a hundred times, and yet it is to be infinite' (Sve. Up. V, 9), which at first states the soul to be atomic and then teaches it to be infinite. Now this is appropriate only in the case of the atomicity of the soul being metaphorical while its infinity is real; for both statements cannot be taken in their primary sense at the same time. And the infinity certainly cannot be understood in a metaphorical sense, since all the Upanishads aim at showing that Brahman constitutes the Self of the soul.--The other passage also (Sve. Up. V, 8) which treats of the measure of the soul, 'The lower one, endowed with the quality of mind and the quality of body, is seen small even like the point of a goad,' teaches the soul's small size to depend on its connexion with the qualities of the buddhi, not upon its own Self. The following passage again, 'That small (anu) Self is to be known by thought' (Mu. Up. III, 1, 9), does not teach that the soul is of atomic size, since the subject of the chapter is Brahman in so far as not to be fathomed by the eye, &c., but to be apprehended by the serene light of knowledge, and since moreover the soul cannot be of atomic size in the primary sense of the word. Hence the statement about anutva (smallness, subtlety) has to be understood as referring either to the difficulty of knowing the soul, or else to its limiting adjuncts. Similarly such passages as 'Having by knowledge taken possession of the whole body' (Kau. Up. III, 6), which mention a difference (between the soul and knowledge), must be understood to mean that the soul takes possession of the whole body through the buddhi, its limiting adjunct; or else they must be considered as mere modes of expression, as when we speak of the body of a stone statue. For we have
already shown that the distinction of quality and thing qualified does not exist in the case of the soul.--The statements as to the soul abiding in the heart are likewise to be explained on the ground of the buddhi abiding there.--That also the soul's passing out and so on depend on the limiting adjuncts, is shown by the passage, 'What is it by whose passing out I shall pass out, and by whose staying I shall stay? He sent forth prâna.' &c. (Pr. Up VI, 3, 4). For where there is no passing out, no going and returning are known; for what has not left the body cannot go and return 1.--As thus the soul (as long as involved in (the samsâra) has for its essence the qualities of its limiting adjuncts it is spoken of as minute. The case is analogous to that of Brahman (prâgña). Just as in those chapters whose topic is the meditation on the qualified Brahman, the highest Self is spoken of as possessing relative minuteness and so on, because it has the qualities of its limiting adjuncts for its essence (cp. 'Smaller than a grain of rice or barley;' 'He who consists of mind, whose body is prâna,' &c., Kh. Up. III, 14, 2; 3); so it is also with the individual soul.--Very well, let us then assume that the transmigratory condition of the soul is due to the qualities of the buddhi forming its essence. From this however, it will follow that, as the conjunction of buddhi and soul--which are different entities--must necessarily come to an end, the soul when disjoined from the buddhi will be altogether undefinable and thence non-existing or rather non-existing in the samsâra state 2.--To this objection the next Sûtra replies.
45:1 So that the distinction insisted on in Sûtra 20 is not valid
45:2 Katham asativam svarûpena sattvâd ity âsaṅkhyâha samsâritvam veti. Ân. Gi.