As the Nasdaq sank on Monday over fears that DeepSeek, a Chinese company, might undercut the entire American A.I. industry with its free, open-source alternative to ChatGPT, my old J.P. Morgan partner Tom Lee, now the head of research at Fundstrat Global Advisors, went on CNBC to offer some words of advice. Yes, he acknowledged, Nvidia had lost some $600 billion of market value—the largest single-day drop for any company ever—and every A.I.-adjacent tech stock from Meta to Microsoft was being dumped by investors. “Markets don’t like uncertainty,” Tom observed, calmly. “They like moats.” Nevertheless, “it’s an overreaction,” he argued. “I’d be looking at this as an opportunity.”