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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