A placebo is a substance that is not a medicine, but is given to someone who is told it is a medicine. A good example is a sugar pill, rather than a pain pill. The placebo effect occurs when the person responds as if the placebo "medicine" is working. This calls awareness to the major role that the mind plays in the body's physical health. If you believe that you are taking something that will make you feel better, most likely it will. The ideas of "neural top-down control of physiology" indicate that the brain directly regulates all systems in the body - all the way down to the cellular level. Doing a self-guided mental scan of your body (yoga nidra is an excellent way to do this) and infusing all systems and cells with thoughts of love, health, and happiness is a good practice for healthy living. This can be applied to the nonphysical, energetic level as well. Scanning your energetic centers (chakras) and infusing them with positive energy will help to center and to align your energetic being. The mind is a powerful medicine.