They define a republic to be a government of laws, and not of men.

John Adams (1735–1826) American lawyer, Founding Father, statesman, US President (1797–1801)

Essay (1775-03-06), “Novanglus,” No. 7, Boston Gazette
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This series of essays was written by Adams under the pseudonym of "Novanglus" (Latin for "New England") responding to essays from his past friend Daniel Leonard as "Massachusettensis" on colonial leadership and what the proper relationship was between the American colonies and Britain.

Adams credited the concept of the line above to Aristotle, Livy, and specifically to James Harrington (1611-77), who (also crediting Aristotle and Livy) wrote in The Commonwealth of Oceana (1656) of "government [...] is the empire of laws and not of men," "a commonwealth is an empire of laws and not of men," and "a commonwealth is a government of laws and not of men."

Adams later used the term ("government of laws and not of men") in the Massachusetts Constitution, Bill of Rights, Article 30 (1780), enforcing a separation of powers between the executive, judicial, and legislative branches.