  To see the problem posed by the analysis of propositional attitude reports, consider what appears to be a simple principle of reasoning, namely, the Principle of Identity Substitution (this is not to be confused with the Rule of Substitution discussed earlier). If a name, say \(n\), appears in a true sentence \(S\), and the identity sentence \(n=m\) is true, then the Principle of Identity Substitution tells us that the substitution of the name \(m\) for the name \(n\) in \(S\) does not affect the truth of \(S\). For example, let \(S\) be the true sentence ‘Mark Twain was an author’, let \(n\) be the name ‘Mark Twain’, and let \(m\) be the name ‘Samuel Clemens’. Then since the identity sentence ‘Mark Twain = Samuel Clemens’ is true, we can substitute ‘Samuel Clemens’ for ‘Mark Twain’ without affecting the truth of the sentence. And indeed, the resulting sentence ‘Samuel Clemens was an author’ is true. In other words, the following argument is valid: