History shows that liberals need radicals. We need radicals because drastic change against entrenched evil and concentrated power requires personal bravery to the point of obsession. It requires a radical sensibility to look beyond today's limits and imagine what seems sheer impossibility within the current social order. And sometimes it's necessary to break the law to redeem the Constitution. No great social change in America has occurred without radicals, beginning with the struggle to end slavery. Causes that now seem mainstream began with radical, impolite and sometimes civil disobedient protest. But here's where the story gets complicated. Radicals also need liberals. Liberals can write policy proposals to their hearts' content. But unless they are backed by radicalism on the ground, they are playing in a sandbox.