This quote isn’t used much, but from Ayn Rand’s “The Dead End”:

“A nation’s productive—and moral, and intellectual—top is the middle  class. It is a broad reservoir of energy, it is a country’s motor and  lifeblood, which feeds the rest. The common denominator of its members,  on their various levels of ability, is: independence. The upper classes  are merely a nation’s past; the middle class is its future.”