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... surprising things can happen to any one who, when a disagreeable or discouraged thought comes into his mind, just has the sense to remember in time and push it out by putting in an agreeable determinedly courageous one. Two things cannot be in one place.
"Where you tend a rose, my lad, A thistle cannot grow."
This passage provides a sort of abstract of one of the book's central themes: the idea that all illness is psycho-somatic (caused by the mind), and thus one need only think positive thoughts if one is to be well. This idea comes from Christian Science, which holds that negative thoughts are the human error to be found at the root of all disease. One must force out ugly thoughts with agreeable ones, for "two things cannot be in one place." This notion is responsible for both Colin and Mary's wondrous metamorphoses. Once they are thinking of the garden and nature, of Dickon and of their own blossoming friendship, they can no longer concern themselves with their own contrariness or with fear of becoming a hunchback and dying an early death. The rather inane epigraph that concludes the passage compares both the positive thoughts and the transformed Colin and Mary with a rose, which of course refers back to the rebirth and cultivation of the secret garden.