What light is to the eye, that knowledge is to the mind. [from Henry Cowles : The Revelation of John (1871)]
Hi, Sunshine and Gustavo,
I agree in essence with your analysis, Gustavo, and would simply like to share how I parse the sentence. Initially the sentence looked wrong to me, but then I imagined hearing it spoken dramatically, with "that" emphasized, as in an old-fashioned sermon, and everything fell into place for me.
I understand "that" to be a pronoun, which does not necessarily contradict the idea that it is a determiner. Pronouns are categorized as determiners in some varieties of modern generative grammar. The important point, however, for me, is that "knowledge" is not the complement of "that" in the sentence.
I understand "that" to be a pronoun that refers to the free relative clause, "what light is to the eye." (Free relative clauses are noun phrases and can be referred to by pronouns.) The main clause then uses topicalization. Instead of the word order "Knowledge is that to the mind," we have "That(,) knowledge is to the mind.
Knowledge is [what light is to the eye / THAT] to the mind.
Knowledge is ___ to the mind what light is to the eye / THAT.
What light is to the eye / THAT, knowledge is ___ to the mind.
What light is to the eye, THAT knowledge is to the mind.
Normally, in modern speech, we'd use the second version, with the extraposed free relative clause and no "that": "Knowledge is to the mind what light is to the eye." The underlying word order -- "Knowledge is what light is to the eye to the mind" -- is too hard to process. Also natural is the third sentence without "that":
- What light is to the eye, knowledge is to the mind.