If we’re lucky, then someday we’ll look back to October 2019 and say that’s when the recovery began. If we’re lucky, we’ll look back to a decision that Twitter CEO Jack Dorsey made, and recognize that at that single moment, sanity began to return. That decision was Dorsey’s resolve not to sell political ads on Twitter. If we’re lucky, that critical decision will be followed by other social media platforms. And someday we’ll look back and recognize just how important that corporate self-restraint was.

Because it is so easy for thinking to get swallowed by the mindless slogans of the day. Mark Zuckerberg tells America that Facebook won’t ban political ads, even just before an election, because Facebook believes in “free speech” — by which Zuckerberg means the speech that will earn Facebook more than $400 million in ad revenue in the 2020 election.

But Dorsey doesn’t oppose free speech. Dorsey instead recognizes how his extraordinary platform can sometimes do more harm than good. By giving political advertisers the tools to buy speech that Twitter has to slice and dice an incredibly digitally active public, political advertisers have the ability to divide us and anger us and render us just nuts. Those tools make it possible to drive certain people away from the polls. They make it possible to terrify other people about going to the polls. Those tools give the clever a million hacks to defeat democracy — especially the clever souls from foreign nations who are not at all interested in American democracy succeeding.

That’s not because advertising, in general, is awful — though I’m sure most people will look at most web pages and long for the days when advertising wasn’t trying to grab every bit of a user’s attention. Instead, in the proper context, for a proper purpose, advertising is just fine. It’s really wonderful if Twitter’s algorithms figure out that I’m likely to be interested in a post about Iceland. I love Iceland. I long to travel there whenever I can. Ads that help me get what I want to get in a market or commercial context are precisely why advertising was invented. And it is why technologies for helping spread the ability to advertise in platforms like Twitter are not inherently evil.

But there is a time and a place for everything. And if the last five years have taught us anything, they have shown us just how much trouble can be caused by sophisticated advertising algorithms in the context of political campaigns.

We still speak of political debate and deliberation on the model of the Lincoln/Douglas debates — where serious thought was matched by serious thought, and a public weighed that thought carefully to decide which was more persuasive. But ads on social media platforms don’t do anything like that. Social media ads are meant to anger or engage; they help divide so as to conquer; they help turn out one kind of vote while suppressing another. I don’t believe the data support the claim that Facebook and Twitter elected Donald Trump. But I do believe that there are plenty of examples of ad-driven social media political campaigns driving the public to do what the public, upon reflection, didn’t want to do.

The best example maybe Brexit. In the weeks leading up to the Brexit vote, the Remain campaign was consistently ahead. Most engaging in that campaign did so in a very traditional way. The Remain campaign was quintessentially traditional. And much of the Leave campaign was as well.

But a faction of the Leave campaign, Leave.EU, leveraged money and Cambridge Analytica to build massive databases that measured the psychological profile of social media users. And then using the models that that massive data had helped build, they spent endless money dividing and angering and suppressing and turning out the votes for their leaving. These techniques are not hidden. Indeed, the victors now brag about them. Just read the diary of the architect of the Brexit victory, a man who declares he had “enjoyed a life of happy anonymity flogging car insurance in Bristol” before “sinking £8 million [$10.4 million] of his personal fortune into a mad-cap campaign” designed to sow “mischief” and “mayhem” in the “guerrilla war” to win Brexit. This had nothing to do with getting a nation to resolve what was best for the nation. It was instead the hacking of the body politic to get it to do what on reflection most polls show Britain wishes it hadn’t done.

Dorsey has obviously looked at these data, and decided to act in a way that both supports democracy and ultimately supports his own platform. No doubt, he could have, like Zuckerberg, simply taken the money. No doubt, there is a lot of money that he has now left on the table. But it is completely sensible, both from a business and democratic perspective, for him to look to the long-term value of his social network — a value fundamentally affected by the games people play with it — and decide that Twitter is better off taking the high road. It is hard to remember, but there really was a time when Facebook was thought to be an obvious good. But under Zuckerberg’s leadership, the company has cut corners and worse, all to the end of making the shareholders, in the short term, richer. But the consequence of that strategy now is that Facebook has faded as a platform of choice; it is no longer “cool,” as any kid will tell you, and it is openly scorned by so many others.

Social media platforms should be places where we express our political views. And if our views are compelling or interesting or important or funny, then it is a wonderful thing if our friends or others share them. Organic growth on a social media platform is a measure of the organic strength of the idea being spread. And in a context unchecked by truth or editors, we should avoid giving anyone an opportunity to leverage money to hack influence, and hence hack democracy.

Advertising on social media platforms is the hack of the day. Twitter deserves great respect for shutting down that hack on its platform. Facebook should follow Twitter’s lead — at least for campaigns with a national profile and hence vulnerable to foreign hacking. And if it doesn’t, then here’s another mindless slogan that Mark Zuckerberg might consider: You reap what you sow, and the poison your platform sows now is going to haunt your platform until a competitor drives you under. It will happen. It always has. What’s surprising is how oblivious this great company seems to be to this obvious if eternal truth.

Lawrence Lessig is the Roy L. Furman Professor of Law and Leadership at Harvard Law School. His latest book, They Don’t Represent Us (2019), is published by Dey Street Books. He’ll speak at the National Constitution Center on Nov. 20 at 6:30 p.m. More event information at constitutioncenter.org.