Final answer:
Sieyes saw the Third Estate as the true support of society, bearing the economic weight and tax burden, and argued for its political recognition inspired by the Enlightenment ideas of liberty, equality, and fraternity.
Step-by-step explanation:
Emanuel Sieyes believed that the Third Estate was the one that "sustain[s] society" because it consisted of the overwhelming majority of the French people who were responsible for the economic productivity and the payment of taxes. The Third Estate included a wide array of social classes from peasants to the burgeoning bourgeoisie, who together faced the brunt of the tax burden and upheld the economic framework of the nation.
While the First and Second Estates held most of the wealth, power, and land, they were exempt from taxes and contributed little to the actual functioning of societal infrastructure.
The Enlightenment's spread of ideas like liberty, equality, and brotherhood, along with the Third Estate's crucial economic role, led to questioning the fairness of the social and political system that favored the aristocracy and clergy. Sieyes saw the Third Estate as embodying the true will of the people, as they were the ones effectively supporting the nation.