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  "body": "\n## Summary\n\nA theory in a first-order language is a set of sentences of that language.\nIts sentences play the role of axioms, such as the axioms of groups or of fields.\nTheories are the syntactic side of model theory, and the structures in which all their sentences hold are their models.\n\n## Explanations\n\n### L1 human\n\nA theory is a list of rules written in a formal language.\nFor example, the three group axioms, written with the symbols $\\cdot$, $e$ and ${}^{-1}$, form the theory of groups.\nA theory does not say which objects satisfy the rules.\nThe objects that satisfy all the rules, here all groups, are the models of the theory.\n",
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