Admit it. You know a nearly 800-point decline in the Dow Jones Industrial Average is only a 3% drop. But it makes you feel uneasy anyway—especially when it happens twice in a row during intraday trading, as it did this week.

It may also make you want to do something. Or at least talk about it with a friend.

So let me tell you a story.

One of the most extraordinary investors I’ve ever met, William “Jack” Hurst, who died in 2014, knew how to handle turbulent markets with masterful calm. By the time I met him in 2005, Mr. Hurst had been almost entirely paralyzed by amyotrophic lateral sclerosis; he could move only a few muscles in his face…

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