There's a proof. The theorem is called the "First Welfare Theorem". There's also...

QuesnayJr on Dec 10, 2019 | parent | context | favorite | on: Quadratic Payments: A Primer

There's a proof. The theorem is called the "First Welfare Theorem". There's also a "Second Welfare Theorem" that says that you can achieve any Pareto efficient outcome through a perfectly competitive market and lump-sum taxes.