Why did Adi Sankaracharya compare the color of Ishwara’s eyes to that of monkey’s nates?
First of all, here is the Chandogya Upanishad verse in question:
tasya yathā kapyāsaṃ puṇḍarīkamevamakṣiṇī
tasyoditi nāma sa eṣa sarvebhyaḥ pāpmabhya udita
udeti ha vai sarvebhyaḥ pāpmabhyo ya evaṃ veda ॥
Whose eyes are like blue lotus's, his name is ut, for he has risen (udita) above all evil. He also who knows this, rises above all evil.
Now Adi Shankaracharya interprets the word "kapyasam" in that verse as saying that the supreme lord's eyes are the color of a monkey's butt, as described in this excerpt from his Chandogya Upanishad Bhashya:
"Kapyasa" is the "asa", seat, of the "Kapi", monkey; the term "asa" being derived from the root "asa", to sit, with the ghan affix. The term "Kapyasa" therefore stands for the part of the monkey's back on which it sits; so that when the lotus is spoken of as "Kapyasa", "monkey-seat", what is meant is that it is extremely bright (and red); so the eyes of the Solar Person also are bright red. Inasmuch as the Simile is an indirect one, - the lotus being likened to the monkey's seat, and the eyes being likened to the lotus, - it cannot be regarded as incompatible with the dignity of the subject.
In any case, to answer your question, no, other commentators do not interpret the word "kapyasam" as referring to a monkey's butt. In fact, this verse played a major role in the history of Sri Vaishnavism. Before Ramanujacharya became a Sri Vaishnava, he was the shishya of a guru named Yadava Prakasha, who was teaching his students Adi Shankaracharya's Chandogya Upanishad Bhashya. When they got to the part of the Bhashya that we're discussing, boiling hot tears came out of Ramanujacharya's eyes. He explained "How can you compare Sriman Narayana's eyes to a monkey's butt? The Chandogya Upanishad is saying that his eyes are the color of a lotus flower kissed by the sun!" Yadavaprakasha was angered that his student was challenging him, and this created a rift between guru and shishya that ultimately led to Ramanujacharya leaving Yadava Prakasha and becoming a Sri Vaishnava.
In any case, here is how the Sri Vaishnava commentator Ranga Ramanuja interprets the verse in this excerpt from his commentary on the Chandogya Upanishad:
Kapi means one who drinks water (kam pibati), namely the sun. The lotus which is thrown out (asyate), that is, struck on and opened by the sun is therefore called kapyasa. The Author similarly states: "(1) Kapyasa is that which has been struck by the sun, for it is endowed with beauty; (2) or again, Kapi means one who drinks water (kam pibati), namely a stalk. Because the lotus) is seated upon a stalk, it) is called kapyasa. This is what is intended here, because the lotus seated on a stalk is more beautiful than one which has been cut off; (3) or, ka refers to water, and the root as (to sit) is preceded by the prefix api. It becomes pyasa) because there is the declaration that the grammarian) Bhaguri holds that in cases where prefixes ava- and api- are added, the first vowel is dropped. Thus) the term kapyasa means that it takes its seat on water, that is, that it lives in water." Taking into account this threefold meaning thus established for the word kapyasa) by the Author, Bhagavat Bhasyakara (= Ramanuja) stated in (his) Vedarthasamgraha" (that it means) "Having pure and large eyes like the petals of a lotus, opened by the sun's beams, and on a fine stalk, born in the deep water." But other meanings such as "like the lotus which resembles the rump of a monkey" were ignored by the Bhasyakara, since they had been ignored by the Author (Vakyakrt) as suffering from such faults as vulgarity of language.
And it's not just people who came after Adi Shankaracharya who disagreed with his interpretation. As I discuss in this question, one of the most important pre-Shankara Vedanta philosophers was the Vakyakara, known as Tanka or Brahmanandi. He wrote an ancient Vakya or commentary on the Chandogya Upanishad, where he analyzed the word "kapyasam" and gave six possible meanings it could have, and then rejected three of them as inappropriate to the context. This excerpt from from S.S. Raghavachar's book "Ramanuja on the Upanishads" gives the six meanings:
Sudarshana Suri, commenting on Ramanuja's explanation, observes that the Vakyakara has enumerated six interpretations of the term, "kapyasam", of which three are acceptable and three are not. All the acceptable views are here set out - opened by the sun, having stout stalks and growing in deep water. The rejected explanations suggest that it refers to the sphere of the sun, the posteriors of a monkey and a state of half-blossoming.
So as you can see, Adi Shankaracharya adopted a meaning which the Vakyakara had earlier rejected as inappropriate to the context.
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