Sen. Bernie Sanders (I-Vt.) isn't buying Elon Musk's vision of a post-money future — at least not without a tax bill to match. On Tuesday, Sanders challenged the world's richest person to support a 5% tax on billionaire wealth, arguing that if Musk truly believes "money won't matter" in 2036, he should put his fortune where his philosophy is.

"If Elon Musk, the richest man alive, really believes 'money won't matter' in 2036, he'd support a 5% tax on his wealth & the wealth of other billionaires to make sure all Americans have a decent standard of living," Sanders posted on X. "The fact that he doesn't tells you everything you need to know."

The exchange is the latest in a running debate about wealth, inequality, and the future of work in an AI-driven economy. It started last week when Musk, CEO of Tesla Inc. (TSLA) and Space Exploration Technologies Corp (SPCX), sat down with The Economist and painted a picture of a world where artificial intelligence and robots make goods and services so abundant that they exceed what people can consume. In that world, Musk suggested, money could become far less important.

When asked how people displaced by AI would earn a living, Musk had a simple answer: the Treasury could just issue checks. He added that the economy could face deflation rather than inflation if production grows faster than the money supply.

It's a provocative vision, and it didn't take long for critics to point out the apparent contradiction between Musk's futuristic utopianism and his real-world opposition to wealth taxes. On Monday, 2024 Nobel Prize-winning economist Daron Acemoglu urged Musk to pledge his fortune to charity by 2036 if he truly believes money will no longer matter. Musk responded, "I am actually going to do something along these lines!"

But Sanders wants more than a vague promise. His call for a 5% wealth tax is a concrete policy proposal that would directly affect Musk, whose net worth Forbes estimates at about $729.8 billion. Musk briefly became the world's first trillionaire in June after SpaceX went public, but lost that status as the stock pulled back from its highs.

Musk has a long history of opposing wealth taxes. He's said he paid more than $10 billion in taxes in a single year and expects to pay "trillions" over his lifetime. Last week, he argued that taxing billionaires alone wouldn't solve the U.S. deficit, suggesting the real solution would require "taxing the living daylights out of everyone."

So far, Musk hasn't responded directly to Sanders's challenge. But the back-and-forth highlights a broader tension: as AI and automation reshape the economy, the question of how to distribute the gains — and who should pay for the transition — is only getting louder.