Abbé Sieyès' pamphlet 'What is the Third Estate?' is a political document published in 1789 that argued for the significance of the Third Estate in French society, which comprised the common people. The pamphlet played a crucial role in shaping revolutionary thought by asserting that the Third Estate represented the true strength of the nation, while the First and Second Estates (clergy and nobility) were essentially useless. This work significantly influenced the events leading up to and during the French Revolution by rallying support for the demands of the common people.