The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
10. And on account of the statement of the assumption (of a metaphor) there is nothing contrary to reason (in agâ denoting the causal matter); just as in the case of honey (denoting the sun) and similar cases.
The word agâ neither expresses that fire, water, and earth belong to the goat species, nor is it to be explained as meaning 'unborn;' it rather expresses an assumption, i.e. it intimates the assumption of the source of all beings (which source comprises fire, water, and earth), being compared to a she-goat. For as accidentally some she-goat might be partly red, partly white, partly black, and might have many young goats resembling her in colour, and as some he-goat might love her and lie by her, while some other he-goat might leave her after having enjoyed her; so the universal causal matter which is tri-coloured, because comprising fire, water, and earth, produces many inanimate and animate beings similar to itself, and is enjoyed by the souls fettered by Nescience, while it is abandoned by those souls which have attained true knowledge.--Nor must we imagine that the distinction of individual souls, which is implied in the preceding explanation, involves that reality of the multiplicity of souls which forms one of the tenets of other philosophical schools. For the purport of the passage is to intimate, not the multiplicity of souls, but the distinction of
the states of bondage and release. This latter distinction is explained with reference to the multiplicity of souls as ordinarily conceived; that multiplicity, however, depends altogether on limiting adjuncts, and is the unreal product of wrong knowledge merely; as we know from scriptural passages such as, 'He is the one God hidden in all beings, all-pervading, the Self in all beings,' &c.--The words 'like the honey' (in the Sûtra) mean that just as the sun, although not being honey, is represented as honey (Kh. Up. III, 1), and speech as a cow (Bri. Up. V, 8), and the heavenly world, &c. as the fires (Bri. Up. VI, 2, 9), so here the causal matter, although not being a she-goat, is metaphorically represented as one. There is therefore nothing contrary to reason in the circumstance of the term agâ being used to denote the aggregate of fire, water, and earth.