Bhashya
रसः कारणं हिरण्यगर्भविज्ञानात्मा चेतन इति केचित् ; तत्र च किल हिरण्यगर्भविज्ञानात्मनः कर्म वाय्वन्तरिक्षयोः प्रयोक्तृ; तत्कर्म वाय्वन्तरिक्षाधारं सत् अन्येषां भूतानां प्रयोक्तृ भवति; तेन स्वकर्मणा वाय्वन्तरिक्षयोः प्रयोक्तेति तयोः रसः कारणमुच्यत इति ।तन्न मूर्तरसेन अतुल्यत्वात्; मूर्तेभ्य तु भूतत्रयस्य रसो मूर्तमेव मण्डलं दृष्टं भूतत्रयसमानजातीयम् ; न चेतनः; तथा अमूर्तयोरपि भूतयोः तत्समानजातीयेनैव अमूर्तरसेन युक्तं भवितुम्, वाक्यप्रवृत्तेस्तुल्यत्वात्; यथा हि मूर्तामूर्ते चतुष्टयधर्मवती विभज्येते, तथा रसरसवतोरपि मूर्तामूर्तयोः तुल्येनैव न्यायेन युक्तो विभागः; न त्वर्धवैशसम् ।मूर्तरसेऽपि मण्डलोपाधिश्चेतनो विवक्ष्यत इति चेत्- अत्यल्पमिदमुच्यते, सर्व-रारम्भकस्य कार्यस्य एष रसः सारः ; तेन हि सारेण सार-वदिदं शरीरं समस्तम्- यथा अधिदैवतभादित्यमण्डलेन ;प्राथम्याच्च- चक्षुषी एव प्रथमे संभवतः संभवत इति,'तेजो रसो निरवर्तताग्निः' इति लिङ्गात् ; तैजसं हि चक्षुः;एतत्सारम् आध्यात्मिकं भूतत्रयम् ; सतो ह्येष रस इतिमूर्तत्वसारत्वे हेत्वर्थः ॥
In plain words
Some say 'the essence is the cause' means Hiraṇyagarbha, that cognizing being, must be conscious. Here's their reasoning: Hiraṇyagarbha's own action works wind and space like instruments; grounded there, that same action goes on to work the other elements too; and because he operates wind and space through his own doing, he — as their essence — gets called their cause. But that doesn't hold up, because it breaks the pattern essence follows on the formed side. Among the formed elements, the essence is itself something formed: the visible disc, the very same kind of thing as the elements it belongs to — and it isn't conscious. So to keep the comparison honest, the essence of the two formless elements has to be formless too, of that same kind, or the whole analogy falls apart. Just as the formed and the formless both carry the same four marks, the split between essence and what carries that essence has to follow the identical rule on both sides — you can't apply it to one and quietly drop it for the other. And if someone says, 'but even the formed essence, the sun, is really a conscious being wearing the disc as its costume' — that misses far too much. This sun is the essence, the very core, of everything that is ever produced; it's by that essence that this whole body has any substance at all, the way the solar disc gives the deity-level world its substance. And there's a reason it comes first: the eyes are the very first organs to appear, on the strength of the line 'from the essence of light, Agni arose' — because the eye truly is a thing of fire and light. This same sun is the essence of the three elements inside the body too, exactly as outside it. And 'for this is indeed the essence of the real' is precisely why it counts as both formed and as essence.
Close to the Sanskrit
'The essence is the cause' — [meaning] Hiraṇyagarbha, whose nature is cognition, is a conscious being — so say some. And here, indeed, the action of Hiraṇyagarbha, the cognizing being, operates wind and space as its instruments; that action, grounded in wind and space, goes on to operate the other elements too; because he operates wind and space through his own action, he — their essence — is said to be their cause. That does not hold, because it doesn't match how essence works with the formed. Among the formed elements, the essence of the trio of elements is itself formed: the visible orb, the same kind of thing as the trio of elements; and it is not conscious. So, likewise, the essence of the two formless elements too must be of that same kind, formless, because the analogy has to run consistently. For just as the formed and the formless are each divided by possessing the four properties, so too, for essence and what possesses that essence, the division between formed and formless must be made by the very same rule; it must not be a half-measure. If someone objects that even in the case of the formed essence, a conscious being wearing the disc as its costume is what's really meant — that reply says far too little: this sun is the essence, the very core, of every effect that is ever produced; for by that essence, this whole body has its substance — just as, at the level of the deities, the solar disc gives it substance. And because it comes first — the eyes indeed are the very first organs to appear — on the evidence of the citation: 'from the essence of light, Agni was produced'; for the eye truly is a thing of fire and light. This same sun is the essence, on the individual level too, of the trio of elements; and 'for this is indeed the essence of the existent' is exactly what gives the reason for its being both formed and being the essence.