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  "body": "\n## Summary\n\nIn mathematical logic, a **predicate variable** is a predicate letter which functions as a \"placeholder\" for a relation (between terms), but which has not been specifically assigned any particular relation (or meaning). Common symbols for denoting predicate variables include capital roman letters such as $P$, $Q$ and $R$, or lower case roman letters, e.g., $x$. In [[first-order logic|086QKPF0A6]], they can be more properly called metalinguistic variables. In higher-order logic, predicate variables correspond to propositional variables which can stand for well-formed formulas of the same logic, and such variables can be quantified by means of (at least) second-order [[quantifiers|0FYQMKFE77]].\n",
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