#artificial-intelligence #geb #goedel-escher-bach #hofstadter #typographical-number-theory
Rule of Specification: Suppose u is a variable which occurs inside the string x. If the string \(\forall u:x\) is a theorem, then so is x, and so are any strings made from x by replacing u, wherever it occurs, by one and the same term. (Restriction: The term which replaces u must not contain any variable that is quantified in x.)