You don’t need a firewall unless there’s a fire, and a fire is precisely what the candidacy of Bernie Sanders has set off among disaffected Democrats.

His message is clear and resonant — that we must rein in big business and stop their unfair practices, embrace some common-sense measures as universal rights — like access to health care, paid family leave and free public college to all — curtail the corruptive influence of big money on government, and reverse the trend of income inequality.

It is hard for liberals to argue with this as a statement of principle. The only question is, “How?” For some, the answers are unsatisfactory, particularly when considering the political realities of an intransigent Congress that has attempted to block the current president at every turn.

But saying to people who believe in Sanders’s vision that it is a mirage is injurious to their sense of wonder and determinism. It says: Stop believing that the impossible is possible. That lands like a wet blanket. It’s antithetical to the American ethos. This country’s lore and its image of its own greatness is rooted in doing what had never been done, what no one thought could be done.