6.23 If two expressions are combined with the identity sign, this means that they are substitutable for one another. But whether this is the case must be evident in the two expressions themselves.

It is a characteristic of the logical form of two expressions that they are substitutable for one another.

What is meant by an expression here? Is it evident in the expression “4” that it is identical with the expression “2 = 2”? Perhaps “expression” means “expression rightly and fully understood”.

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