Everything we know about the world comes to us through our senses.Traditionally , we were thought to have just five of them — sight , hearing , touch , smell and taste. Although we pay little attention to them , each of these senses is precious and almost irreplaceable — as we discover , to our sorrow , if we lose one.People usually fear blindness above all other disabilities.Yet deafness can be an even more severe handicap( 缺陷 ) , especially in early life , when children learn language.This is why Helen Keller's achievements were so extraordinary. As a result of an illness at the age of 19 months , she lost both sight and hearing and sank into a totally dark , silent world.She was rescued from this terrible separation by her teacher , Anne Sullivan , who managed to explain , by tapping signs into the little girl's palm , that things have names , that letters make up words , and that these can be used to express wants or ideas. Helen Keller later grew into a writer and a well ­ known advocate for the handicapped.Her remarkable development owed a great deal to her determination , her teacher , and her family.But it also showed that when a sense (or two , in Helen Keller's case) is missing , another sense (in her case , touch) may be trained to make up for the loss , at least in part. Our nervous system reacts only to a selected range of wavelengths , vibrations , or other properties.It is limited by our genes , as well as our previous experience and our present state of attention. What draws our attention , in many cases , is change.Our senses are finely adjusted to change. Stationary or unchanging objects become part of the scenery and are mostly unseen.Usual sounds become background noise , mostly unheard.The feel of a sweater against our skin is soon unnoticed.