The difference between these two kinds of liberalism — constitutionalism grounded in humility about human nature and progressivism grounded in utopian expectations — is a crucial fault line of our politics, and has divided the friends of liberty since at least the French Revolution. It speaks to two kinds of views about just what liberal politics is.One view, which has always been the less common one, holds that liberal institutions were the product of countless generations of political and cultural evolution in the West, which by the time of the Enlightenment, and especially in Britain, had begun to arrive at political forms that pointed toward some timeless principles in which our common life must be grounded, that accounted for the complexities of society, and that allowed for a workable balance between freedom and effective government given the constraints of human nature. Liberalism, in this view, involves the preservation and gradual improvement of those forms because they allow us both to grasp the proper principles of politics and to govern ourselves well.
The other, and more common, view argues that liberal institutions were the result of a discovery of new political principles in the Enlightenment — principles that pointed toward new ideals and institutions, and toward an ideal society. Liberalism, in this view, is the pursuit of that ideal society. Thus one view understands liberalism as an accomplishment to be preserved and enhanced, while another sees it as a discovery that points beyond the existing arrangements of society. One holds that the prudent forms of liberal institutions are what matter most, while the other holds that the utopian goals of liberal politics are paramount. One is conservative while the other is progressive.
The principles that the progressive form of liberalism thought it had discovered were much like those that more conservative liberals believed society had arrived at through long experience: principles of natural rights that define the proper ends and bounds of government. Thus for a time, progressive and conservative liberals in America — such as Thomas Jefferson and Thomas Paine on one hand and James Madison and Alexander Hamilton on the other — seemed to be advancing roughly the same general vision of government. But when those principles failed to yield the ideal society (and when industrialism seemed to put that ideal farther off than ever), the more progressive or radical liberals abandoned these principles in favor of their utopian ambitions. At that point, progressive and conservative American liberals parted ways — the former drawn to post-liberal philosophies of utopian ends (often translated from German) while the latter continued to defend the restraining mechanisms of classical-liberal institutions and the skeptical worldview that underlies them.
That division is evident in many of our most profound debates today, and especially in the debate between the Left and the Right about the Constitution