It’s not always as easy as asking, ‘Why not?’

In George Bernard Shaw’s “Back to Methuselah,” the serpent says to Eve in the Garden of Eden: “You see things; and you say, ‘Why?’ But I dream things that never were; and I say, ‘Why not?’”

What’s funny is that a version of this quote is often ascribed to Robert F. Kennedy as the perfect distillation of progressive idealism.

I don’t think Kennedy’s idealism was the root of all evil, or even remotely evil in its intent. But this idea — on display at the Democratic convention last week and an article of faith in Democratic politics — that all that prevents us from doing profound, “transformative” things is our lack of imagination can have objectively evil consequences.

Many of the Democrats who sought their party’s nomination this year didn’t just promise to socialize medicine, enact the Green New Deal, ban fracking, etc. They also insisted that it would be fairly easy to do. The only things that prevented such transformational policies were obstruction by villainous billionaires or special interests and the failure of previous politicians to think big enough.

More recently, progressive activists and politicians have endorsed the idea of abolishing the police and “canceling” rent.

The first problem with this sort of rhetoric is practical. Many of these proposals are simply bad ideas on the merits. Canceling rent and abolishing the police are catastrophically dumb ideas. If you cancel rent, landlords will be unable to pay for heat or electricity, never mind repairs. Countless plumbers, electricians, janitors, painters, et al., would lose their livelihoods. Landlords would default on their mortgage payments to banks. And banks would be stuck with properties that are essentially worthless, causing a cascading economic crisis that would make our current travails seem puny by comparison. And that’s if the courts let any of this happen in the first place.

Truly abolishing the police would invite horrifying levels of violence, looting and robbery, as well as a riot of vigilantism as homeowners, business owners and other citizens took self-defense into their own hands.

The consequences of the Green New Deal or Medicare for All wouldn’t be nearly as biblical, but reasonable people can still acknowledge they would involve massive trade-offs. Whether you think those trade-offs are worth it speaks volumes about where you stand politically.

Even if you think it would be worth the effort to destroy the fossil fuel industry and lay waste to the health care system as we know it, spending trillions of dollars borrowed from China, what can’t be denied by serious people is that it would take enormous political effort over many years and many elections.

In other words, even if the Green New Deal or Medicare for All are necessary and desirable, it’s grotesquely irresponsible to pretend that a single president or congressional majority could deliver them. Barack Obama moved heaven and earth to get the Affordable Care Act passed, and he barely did it. Doing so came at the cost of the rest of his legislative agenda, Democratic control of Congress, and the jobs of more than 1,000 elected officials of his own party.

Many wonderful things have come from visionaries asking, “Why not?” — airplanes, antibiotics, vaccines, nuclear power. But the difference between scientific breakthroughs and political transformations is profound. Science begins on paper and methodically builds upward from the drawing board. The sorts of transformations politicians promise, including President Donald Trump, try to work backward from the desired result. Science begins with A and works its way to Z. The zealots want to start with Z and work their way back to A later.

Our constitutional system was set up to work the other way around — to start small at the local level, letting successes build upward by the power of example and the building of consensus. In recent years, though, we have become convinced that the transformation trickles down from the president rather than up from the people.

What makes this approach so dangerous is that people believe it can work. The zealots tell voters, “All you have to do is elect me or my team, and your work is done.” Then, when reality bites back, the politicians and activists don’t say, “Hey, I was wrong. This is really hard.” They don’t blame the hard work required to forge compromise implicit in a democratic system; they blame the sinister forces — billionaires, globalists, greedy corporations, etc. — that stymied the revolution from above. And that makes sense if you actually believed their agenda was easy and cost-free in the first place.

The serpent asks, “Why not?” because he knows the question fuels chaos and anger. The politicians ask, “Why not?” because they know not what they do.