Be careful of your thoughts. Whatever you send out of your mind, comes back to you. Every thought you think, is a boomerang. If you hate another, hate will come back to you. If you love others, love will come back to you. An evil thought is thrice cursed. First, it harms the thinker by doing injustice to his mental body. Secondly, it harms the person who is its object. Lastly, it harms all mankind by vitiating the whole mental atmosphere.
Every evil thought is as a sword drawn on the person to whom it is directed. If you entertain thoughts of hatred, you are really a murderer of that man against whom you foster thoughts of hatred. You are your own suicide, because these thoughts rebound upon you only. A mind tenanted by evil thoughts acts as a magnet to attract like thoughts from others and thus intensifies the original evil. Evil thoughts thrown into the mental atmosphere poison receptive minds. To dwell on an evil thought gradually deprives it of its repulsiveness and impels the thinker to perform an action which embodies it.
Very carefully watch all your thoughts. Suppose you are assailed by gloomy thoughts. You experience depression. Take a small cup of milk or tea. Sit calmly. Close your eyes. Find out the cause for the depression and try to remove the cause. The best method to overcome the gloomy thoughts and the consequent depression, is to think of inspiring thoughts and inspiring things. Remember again, positive overcomes negative. This is a grand effective law of nature. [...]
The science of thought power is very interesting and subtle. This thought-world is more real relatively than this physical universe. The power of thought is very great. Every thought of yours has a literal value to you in every possible way. The strength of your body, the strength of your mind, your success in life and the pleasures you give to others by your company - all depend on the nature and quality of your thoughts. You must know thought-culture, and develop thought power.
--Swami Sivananda, in 'Thought Power'