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Hold thought steadfastly to the enduring, the good, and the true, and you will bring these into your experience proportionably to their occupancy of your thoughts.
(Science and Health with Key to the Scriptures by Mary Baker Eddy p261:4)
The Importance of Our Thoughts 2 comments
Christian Science Canberra 
So true. Watch that thinking, and keep your minds so full of Truth, Life and Love, that sin sickness and death cannot enter in, Mrs Eddy says.
We are the only one who can control our thoughts. It is possibly the only thing we have any control over. It is really hard to do when confronted with so much crap in the world though. But wait, does crap really exist or does one only think crap exists? Is it a case of ‘you get what you believe’?