No clear line separates mental health from mental illness.
Symptoms help classify mental disorders.
But some disorders with similar symptoms may have different causes. An imbalance of brain chemistry isn't the cause of all mental disorders. Genetics and environmental conditions also may be factors. For example, a person may inherit a genetic predisposition toward depression and may experience depression even though no specific event triggers it. On the other hand, a person who has no inherited tendency toward depression may suffer a loss such as the death of a loved one that triggers symptoms of depression.