After a tornado or similar disaster, some people occasionally try to find a silver lining: At least, they speculate, the event will provide jobs for construction crews, window companies, auto-body shops, and so on. Those beneficiaries will then spend their new income with other businesses, and everybody will gain.

The assumption is a common one -- and a mistaken one, as the French political economist Frederic Bastiat explained in an 1850 essay.

In "That Which Is Seen and That Which Is Unseen," Bastiat writes of a shopkeeper whose window is broken. While onlookers see the benefit to the man who repairs the window, they do not see the hidden costs: Buying a new window costs the shopkeeper the opportunity to buy something else: "If he had not had a window to replace, he would, perhaps, have replaced his old shoes, or added another book to his library. In short, he would have employed his six francs in some way, which this accident has prevented."

Then he would have had both a window and a new pair of shoes or a new book, instead of just a window. Likewise, the person whose car has been wrecked by a tornado will spend several thousand dollars to fix it, and eventually have a working car again. But without the tornado he could have spent the money on a semester at college or new bedroom furniture. Then he would have had both a working car and college credits or new furniture.

Tornadoes are called natural disasters for good reason.