Salus populi suprema lex esto
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Salus populi suprema lex esto (Latin "Let the good of the people be the supreme law" or "The welfare of the people shall be the supreme law") is a state motto of Missouri, accepted, like many other states, as an element of its state seal. The source apparently can be found in Cicero's "De Legibus" (book III, part III, sub. VIII).
John Locke, in his Second Treatise on Government, refers to it as a fundamental rule for government.