I believe it to be true, and live in that truth. Aside from all the religious and spiritual admonitions, the latest science also confirms this.
There is a neural center in our hearts - a bunch of brain cells in our hearts. When we calm the breathing, focus on a sense of warmth or opening in our hearts, and set an intention to love, this center activates. Anyone can do it, and it takes about 30 seconds.
When we do it: heart rate settles, blood pressure stabilizes, the heart generates a smooth, even magnetic field larger than the body, and all neural and hormonal systems come quickly into balance.
Then the neural center in the heart sends a message to the prefrontal cortex (classically called the third eye. It is a message encouraging unification and love, within ourselves and in relation to the people and the world around us.
All this ties directly to the more normal meaning of "as a man thinketh in his heart." Angry, hateful thoughts, even irritation, shut down this heart center. To think a cursing thought does *to us* what doing murder does *to us,* as Jesus teaches.
Let me be clear. I appreciate those who keep the outer commandment / precept not to kill.
But I also encourage all of us to follow the inner commandments: To love ourselves in a healthy way; to love our neighbors as ourselves; and to love our enemies this same way. This unconditional love (agape) heals us and changes our destiny - for the better.