Sidney or Sydney, Algernon, 1622–83, English politician; son of Robert Sidney, earl of Leicester. He served in the parliamentary forces during the English civil war and was a member (1652–53) of the council of state of the Commonwealth, but he opposed the dictatorial rule of Oliver Cromwell. Reappointed (1659) to the council of state, he was abroad at the time of the Restoration (1660) and remained there until 1677, when he returned to England to attend to personal affairs. He soon became associated with the opposition to Charles II, joining Lord William Russell and others in negotiations with French agents and in vague plots for an insurrection, perhaps to place the duke of Monmouth on the throne. His implication in these conspiracies was discovered by the exposure of the Rye House Plot. After a brutal and arbitrary trial by Judge Jeffreys, Sidney was convicted of treason and executed. Sidney's liberal ideals were set forth in his Discourses Concerning Government (1698), a treatise that had great influence on 18th-century political thought, especially in the American colonies.
See biography by A. C. Ewald (2 vol., 1873); J. Scott, Algernon Sidney and the English Republic 1623–1677 (1988).
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