Why is it impossible to stop thinking?

"Why is it impossible to stop thinking, to render the mind a complete blank?" a Scientific American reader wonders.

The answer comes from Barry Gordon, professor of neurology and cognitive science at the Johns Hopkins University School of Medicine.

Constant thinking is what propelled us from being a favorite food on the savanna—and a species that nearly went extinct—to becoming the most accomplished life-form on this planet. Even in the modern world, our mind always churns to find hazards and opportunities in the data we derive from our surroundings, somewhat like a search engine server. Our brain goes one step further, however, by also thinking proactively, a task that takes even more mental processing.