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I read such a sentence from SICP

Allowing quotation in a language wreaks havoc with the ability to reason about the language in simple terms, because it destroys the notion that equals can be substituted for equals. For example, three is one plus two, but the word “three” is not the phrase “one plus two.” Quotation is powerful because it gives us a way to build expressions that manipulate other expressions (as we will see when we write an interpreter in [[Chapter 4]]). But allowing statements in a language that talk about other statements in that language makes it very difficult to maintain any coherent principle of what “equals can be substituted for equals” should mean. For example, if we know that the evening star is the morning star, then from the statement “the evening star is Venus” we can deduce “the morning star is Venus.” However, given that “John knows that the evening star is Venus” we cannot infer that “John knows that the morning star is Venus.”

The part of the common practice:

The common practice in natural languages is to use quotation marks to indicate that a word or a sentence is to be treated literally as a string of characters. For instance, the first letter of “John” is clearly “J.” If we tell somebody “say your name aloud,” we expect to hear that person's name. However, if we tell somebody “say ‘your name' aloud,” we expect to hear the words “your name.” Note that we are forced to nest quotation marks to describe what somebody else might say.

I do not get the idea very clearly, the quotation:

 "John knows that the evening star is Venus”

"John" is not counted in "we" of "if we know that the evening star is the morning star"?

3

The "we" in this text is referring to the author and readers of the text. John is not counted as being among them.

In a bit more detail, these statements are talking about logical constructs. In essence, if "the morning star" is A, "the evening star" is B, and "Venus" is C, then it asserts that A = B, and then says that if A=C, then B=C as well. However, if you're talking about a specific person who knows that A=C, then it doesn't necessarily follow that they also know that B=C - the two are separate facts that a person could know.

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