Dozens of philosophers have since considered the problem, among them George Berkeley, Gottfried Leibniz, Voltaire, Diderot, Adam Smith and William James. And some efforts have been made to answer the question experimentally, beginning in the early 18th century with studies of patients whose congenital cataracts had been removed in adulthood and continuing recently in observations of newborns.

But according to the authors of the new experiment, the studies have been inadequate, never establishing how well the patient could see afterward, or failing to test soon enough after surgery so that the subject was still completely inexperienced with vision.

The new research appears to show definitively that Locke was right. The brain cannot immediately make sense of what the eyes are taking in, and the blind man given the ability to see cannot distinguish the two objects. But he can very quickly learn to do so.

Working with a group that provides medical treatment to the blind and visually impaired in resource-poor countries, the researchers tested five subjects from rural northern India, four boys and a girl ages 8 to 17. A all had been blind since birth, one with a disorder of the cornea, and the others with cataracts. Before their operations they could perceive light, and two could discern its direction, but none could see objects. Afterward, they all had vision measured at 20/160 or better, good enough to distinguish objects and carry out the tasks of daily living.

The children were tested within 48 hours of their operations. The researchers placed 20 small objects similar to Lego blocks on a table where they could be seen, but not touched. Then they had the children feel identical blocks under the table where they were invisible, and try to match them with those they could see. The average performance in matching one object with another by either touch or sight alone was high, close to 100 percent. Yet when they were asked to match an object they had felt with an object seen, the average number of correct answers dropped to barely better than chance.