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The city has always been an engine of intellectual life, from the 18th-century cafes of Lon-don, where citizens gathered to discuss chemistry and politics, to the Left Bank bars of modem Paris, where Picasso talked about modem art. Without the metropolis, we might not have had the great art of Shakespeare.
And yet, city life isn' t easy. Now scientists have begun to examine how the city affects the brain, and the results are depressing. Just being in an urban environment, they have found, im-pairs(损害) our basic mental processes. After spending a few minutes on a crowded city street,the brain is less able to hold things in memory, and suffers from reduced self-control. While it' s long been recognized that city life is exhausting, this new research suggests that cities actually dull our thinking, sometimes dramatically so.
One of the main forces at work is a complete lack of nature, which is surprisingly beneficial for the brain. Studies have demonstrated, for instance, that hospital patients recover more quickly when they can see trees from their windows, and that women living in public housing are better able to focus when their apartments overlook a lawn. Even these glimpses of nature improve brain performance, it seems, because they provide a mental break from the urban life.
This research arrives just as humans cross an important milestone(里程碑). For the first time in history, the majority of people live in cities. Instead of inhabiting wide-open spaces, we' re crowded into concrete jungles, surrounded by traffic and millions of strangers. In recent years, it' s become clear that such unnatural surroundings have important implications for our mental and phys-ical health, and can powerfully alter how we think.
This research is also leading some scientists to dabble( 涉足 ) in urban design, as they look for ways to make the city less damaging to the brain. The good news is that even slight alterations,such as planting more trees in the inner city or creating urban parks with a greater variety of plants, can significantly reduce the negative side effects of city life. The mind needs nature, and even a lit-tle bit can be a big help.
 Which of the following is the main idea of this passage?