The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
39. (The prâna is Brahman), on account of the trembling (predicated of the whole world).
The discussion of qualification for Brahma-knowledge--on which we entered as an opportunity offered--being finished we return to our chief topic, i.e. the enquiry into the purport of the Vedânta-texts.--We read (Ka. Up. II, 6, 2), 'Whatever there is, the whole world when gone forth trembles in the prâna. It (the prâna) is a great terror, a raised thunderbolt. Those who know it become immortal 1.'--This passage declares that this whole world trembles, abiding in prâna, and that there is raised something very terrible, called a thunderbolt, and that through its knowledge immortality is obtained. But as it is not immediately clear what the prâna is, and what that terrible thunderbolt, a discussion arises.
The pûrvapakshin maintains that, in accordance with the ordinary meaning of the term, prâna denotes the air with its five modifications, that the word 'thunderbolt' also is to be taken in its ordinary sense, and that thus the whole passage contains a glorification of air. For, he says, this whole world trembles, abiding within air with its five forms--which is here called prâna--and the terrible thunderbolts also spring from air (or wind) as their cause. For in the air, people say, when it manifests itself in the form of Parganya, lightning, thunder, rain, and thunderbolts manifest themselves.--Through the knowledge of that air immortality
also can be obtained; for another scriptural passage says, 'Air is everything by itself, and air is all things together. He who knows this conquers death.'--We therefore conclude that the same air is to be understood in the passage under discussion.
To this we make the following reply.--Brahman only can be meant, on account of what precedes as well as what follows. In the preceding as well as the subsequent part of the chapter Brahman only is spoken of; how then can it be supposed that in the intermediate part all at once the air should be referred to? The immediately preceding passage runs as follows, 'That only is called the Bright, that is called Brahman, that alone is called the Immortal. All worlds are contained in it, and no one goes beyond it.' That the Brahman there spoken of forms the topic of our passage also, we conclude, firstly, from proximity; and, secondly, from the circumstance that in the clause, 'The whole world trembles in prâna we recognise a quality of Brahman, viz. its constituting the abode of the whole world. That the word prâna can denote the highest Self also, appears from such passages as 'the prâna of prâna' (Bri. Up. IV, 4, 18). Being the cause of trembling, moreover, is a quality which, properly appertains to the highest Self only, not to mere air. Thus Scripture says, 'No mortal lives by the prâna and the breath that goes down. We live by another in whom these two repose' (Ka. Up. II, 5, 5). And also in the passage subsequent to the one under discussion, ('From terror of it fire burns, from terror the sun burns, from terror Indra and Vâyu, and Death as the fifth run away,') Brahman, and not the air, must be supposed to be spoken of, since the subject of that passage is represented as the cause of fear on the part of the whole world inclusive of the air itself. Thence we again conclude that the passage under discussion also refers to Brahman, firstly, on the ground of proximity; and, secondly, because we recognise a quality of Brahman, viz. its being the cause of fear, in the words, 'A great terror, a raised thunderbolt.' The word 'thunderbolt' is here used to denote a cause of fear in general. Thus in ordinary life also a man strictly
carries out a king's command because he fearfully considers in his mind, 'A thunderbolt (i.e. the king's wrath, or threatened punishment) is hanging over my head; it might fall if I did not carry out his command.' In the same manner this whole world inclusive of fire, air, sun, and so on, regularly carries on its manifold functions from fear of Brahman; hence Brahman as inspiring fear is compared to a thunderbolt. Similarly, another scriptural passage, whose topic is Brahman, declares, 'From terror of it the wind blows, from terror the sun rises; from terror of it Agni and Indra, yea, Death runs as the fifth.'--That Brahman is what is referred to in our passage, further follows from the declaration that the fruit of its cognition is immortality. For that immortality is the fruit of the knowledge of Brahman is known, for instance, from the mantra, 'A man who knows him only passes over death, there is no other path to go' (Svet. Up. VI, 15).--That immortality which the pûrvapakshin asserts to be sometimes represented as the fruit of the knowledge of the air is a merely relative one; for there (i.e. in the chapter from which the passage is quoted) at first the highest Self is spoken of, by means of a new topic being started (Bri. Up. III, 4), and thereupon the inferior nature of the air and so on is referred to. ('Everything else is evil.')--That in the passage under discussion the highest Self is meant appears finally from the general subject-matter; for the question (asked by Nakiketas in I, 2, 14, 'That which thou seest as neither this nor that, as neither effect nor cause, as neither past nor future tell me that') refers to the highest Self.
229:1 As the words stand in the original they might be translated as follows (and are so translated by the pûrvapakshin), 'Whatever there is, the whole world trembles in the prâna, there goes forth (from it) a great terror, viz. the raised thunderbolt.'