Abbe Sieyes was the more common name for Emmanuel Joseph Sieyes, a prominent Roman Catholic clergyman who was a chief supporter of the French Revolution. His 1789 pamphlet What is the Third Estate became one of the most influential books of the revolution and helped turn the Estates-General into the National Assembly. He is also important because it was his 1799 coup that brought Napoleon Bonaparte to power.
He was a complex man, because while most of the clergy supported the monarch and was executed along with the royalists as traitors, Abbe was a prominent leader in the revolution, who help shape as steer the revolution which ultimately cost many of his peers their lives. He became distraught with the course the revolution during the Great Terror and melted into the scenery. After the death of Robespierre, he reemerged as a leader, but never achieved what he hoped for, a Constitutional Monarchy.