The ideals of liberalism seem increasingly under threat these days, so it's worth reviewing what they are, where they come from, and why it's essential that they make a comeback. The first step is to recognize that they were not invented by some obsolete English philosopher. Rather, in their most general form, liberal principles have been rediscovered repeatedly and throughout history as practical tools for reconciling two basic social facts:
Many of our deepest moral and metaphysical beliefs, like how to live a good life or which God to worship, are inherently contestable — reasonable people can and will disagree;
We nonetheless all stand to benefit (on our own terms) from a social structure that enables peaceful cooperation.