another one from The Secret Garden

In each century since the beginning of the world wonderful things have
been discovered. In the last century more amazing things were found out
than in any century before. In this new century hundreds of things still
more astounding will be brought to light. At first people refuse to
believe that a strange new thing can be done, then they begin to hope it
can be done, then they see it can be done--then it is done and all the
world wonders why it was not done centuries ago. 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.

- From The Secret Garden

"Where you tend a rose, my lad,
A thistle cannot grow."