"One of the new things people began
             to find out in the last century was that
          thoughts-just mere thoughts-are as powerful
  as electric batteries-as good for one as sunlight is, 
     or as bad for one as poison. To let a sad thought 
or a bad one get into your mind is as dangerous as 
       letting a scarlet fever germ get into your body.
            If you let it stay there after it has got in, 
       you may never get over it as long as you live."

-Frances Hodgson Burnett, The Secret Garden-