x is always equal to x by definition of “equal”.
Turing machines are theoretical models of computers. Everything a normal computer can do can be done with a Turing machine and vice versa (if we have enough storage), but Turing machines are pretty simple in their design so they are often easier to analyze. You can use a Turing machine to prove various statements (e.g. “4 is even”), just like you can do it with a normal computer.
Are you asking about Gödel’s incompleteness theorems? They say (roughly) that not all true statements have a proof.
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