The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
9. But the (elements) beginning with light (are meant by the term agâ); for some read so in their text.
By the term agâ we have to understand the causal matter of the four classes of beings, which matter has sprung from the highest Lord and begins with light, i.e. comprises fire, water, and earth.--The word 'but' (in the Sûtra) gives emphasis to the assertion.--This agâ is to be considered as comprising three elementary substances, not as consisting of three gunas in the Sânkhya sense. We draw this conclusion from the fact that one sâkhâ, after having related how fire, water, and earth sprang from the highest Lord, assigns to them red colour, and so on. 'The red colour of burning fire
[paragraph continues] (agni) is the colour of the elementary fire (tegas), its white colour is the colour of water, its black colour the colour of earth,' &c. Now those three elements--fire, water, and earth--we recognise in the Svetâsvatara passage, as the words red, white, and black are common to both passages, and as these words primarily denote special colours and can be applied to the Sânkhya gunas in a secondary sense only. That passages whose sense is beyond doubt are to be used for the interpretation of doubtful passages, is a generally acknowledged rule. As we therefore find that in the Svetâsvatara--after the general topic has been started in I, 1, 'The Brahman-students say, Is Brahman the cause?'--the text, previous to the passage under discussion, speaks of a power of the highest Lord which arranges the whole world ('the Sages devoted to meditation and concentration have seen the power belonging to God himself, hidden in its own qualities'); and as further that same power is referred to in two subsequent complementary passages ('Know then, Prakriti is Mâyâ, and the great Lord he who is affected with Mâyâ;' 'who being one only rules over every germ;' IV, 10, 11); it cannot possibly be asserted that the mantra treating of the agâ refers to some independent causal matter called pradhâna. We rather assert, on the ground of the general subject-matter, that the mantra describes the same divine power referred to in the other passages, in which names and forms lie unevolved, and which we assume as the antecedent condition of that state of the world in which names and forms are evolved. And that divine power is represented as three-coloured, because its products, viz. fire, water, and earth, have three distinct colours.--But how can we maintain, on the ground of fire, water, and earth having three colours, that the causal matter is appropriately called a three-coloured agâ? if we consider, on the one hand, that the exterior form of the genus agâ (i.e. goat) does not inhere in fire, water, and earth; and, on the other hand, that Scripture teaches fire, water, and earth to have been produced, so that the word agâ cannot be taken in the sense 'non-produced 1.'--To this question the next Sûtra replies.
255:1 Here there seems to be a certain discrepancy between the p. 256 views of the Sûtra writer and Sankara. Govindânanda notes that according to the Bhâshyakrit agâ means simply mâyâ--which interpretation is based on prakarana--while, according to the Sûtra-krit, who explains agâ on the ground of the Khândogya-passage treating of the three primary elements, agâ denotes the aggregate of those three elements constituting an avântaraprakriti.--On Sankara's explanation the term agâ presents no difficulties, for mâyâ is agâ, i.e. unborn, not produced. On the explanation of the Sûtra writer, however, agâ cannot mean unborn, since the three primary elements are products. Hence we are thrown back on the rûdhi signification of agâ, according to which it means she-goat. But how can the avântara-prakriti be called a she-goat? To this question the next Sûtra replies.