As a historian of the rise of the city in American life, he argued that for a full understanding of the Jacksonian democratic movement: "It is necessary to consider the changed circumstances of life of the common man in the new industrial centers of the East since the opening years of the nineteenth century." That was a challenge to the frontier thesis of his Harvard colleague Frederick Jackson Turner. In Schlesinger's essay, the common man of the Mississippi Valley and the common man of eastern industrialism stood uneasily side by side. Schlesinger characterized prejudice against Catholics as "the deepest bias in the history of the American people".