Short answers, pulled from the story.
Emmanuel Joseph Sieyès was a French Catholic priest and political theorist born on the 3rd of May 1748 in Fréjus. His January 1789 pamphlet What Is the Third Estate? became the political manifesto of the Revolution, helping transform the Estates-General into the National Assembly in June 1789.
Sieyès argued that the Third Estate constituted the nation whether or not the privileged orders were present, and that the aristocracy was a fraudulent institution that consumed society's products without contributing to them. He called noble privilege "treason to the commonwealth" and demanded that the Estates-General vote by head rather than by order.
Sieyès was among the chief instigators of the coup of 18 Brumaire on the 9th of November 1799, which dissolved the Directory and installed Napoleon Bonaparte in power. Bonaparte then discarded Sieyès's own constitutional plan and replaced it with the Constitution of the Year VIII, which became the basis of the French Consulate.
When asked what he had done during the Terror, Sieyès famously replied "J'ai vécu," meaning "I lived." He had been menaced by the Terror and abjured his faith at the time of the installation of the Cult of Reason.
Sieyès coined the term "sociologie" in an unpublished manuscript in 1780. The philosopher Auguste Comte used the term approximately fifty years later to name the science of society, which became known in English as sociology. Sieyès was also among the first to use the phrase science sociale.
Sieyès died in Paris in 1836 at the age of 88. After the Second Restoration, Louis XVIII expelled him from the Academy of Moral and Political Sciences in 1816, and he moved to Brussels. He returned to France after the July Revolution of 1830 and lived out his final years in Paris.