Just like the body, a mind can be healthy. The measure of this is called mental health. According to the World Health Organization (WHO), there is not one way to measure mental health in all people, because there are many things in our environment that might make what is mentally healthy different from one person to another. In general, most experts agree that "mental health" and "mental illness" are not opposites. In other words, not having a mental illness does not mean you are in good mental health.