The five senses have traditionally been viewed as distinct yet complementary. Each sense is thought to have its own range of stimuli that are incapable of stimulating the other senses. However, recent research has discovered that some people taste a banana and claim that they are tasting blue, or see a color and say that it has a specific smell. This shows that such people, called synesthesiacs, have senses that do not respect the usual boundaries between the five recognized senses.
The statement 'Synesthesiacs demonstrate a general, systematic impairment in their ability to use and understand words' most seriously weakens the argument.