Steven Pinker, a psychologist at Harvard, recently laid out a deceptively simple idea that might explain why it feels harder to have a conversation across political lines these days.
Common knowledge, he explained, isn't just knowing something — it's knowing that the other person knows it, and knowing that they know you know it, spiraling outward infinitely. It's the bedrock of everything from language to currency to why stock markets move the way they do. Without it, coordination breaks down. Trust erodes.
The problem isn't that we've lost our grip on objective truth, Pinker argued during a conversation at Harvard's Kennedy School. The real issue is subtler: we've fragmented so thoroughly into separate media ecosystems and political tribes that we can no longer establish what counts as common knowledge in the first place. You might know something. I might know something different. But we can't agree on what "we" know together.
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Start Your News DetoxThe cost of intellectual caution
Pinker expressed particular concern about how this plays out in academia, where the pressure to avoid saying the wrong thing has started to override the willingness to test ideas openly. He argued that even objectionable speech should remain part of the conversation, because the academic project — at its core — requires being wrong in public sometimes. You can't evaluate an idea if you're afraid to voice it.
This isn't a call for consequence-free speech. It's a distinction between criticizing someone's ideas and judging their moral worth as a person. The first is how knowledge advances. The second is how intellectual culture calcifies.
Historically, Pinker noted, skepticism about objective truth was the default. People believed in their own experience, their own community's understanding, but the idea that there were verifiable answers to big questions — that required a particular historical moment and a particular faith in institutions and evidence.
The challenge now isn't convincing people that truth exists. It's rebuilding the shared ground where we can even argue about what's true. That requires both intellectual humility (I might be wrong) and intellectual courage (I'm going to say it anyway).
For democracies built on the assumption that citizens can reason together, that shared foundation matters more than any single policy disagreement.
