Dualism is the illusion that there are two unrelated principles in the universe. Me and the other.
To believe that there are two sides of a thing - bright/dark, big/small, light/shadow, right/false - lets us forget how many shades lie between. We tend to consider to one of the two extremes only. Out of this the struggle of the opposing forces arises. Dualistic motivated action always is aggressive and destructive.
Mine is not yours!
An endless series of conflicts follow, for each solution brings new problems. This leads to isolation and violence. Even in the area of medicine ("battlefield" hospital). There is not cured, but "the enemy is fought". The enemy is called illness, or more worse, the enemy is called "sick body". Treatments distroy healthy tissue too (side effects of antibiotics, medicine against pain, chemotherapy). Instead of considering our body as what its wants do be for us: A mediator who shows us what is injured in emotional, spiritual or psychological field. Because a lot of physical signs are expressions of deep-seated sorrows.
Considered in a spiritual way there is no separation, no dualism in nature. In nature everything is "we" and causes alternation. Is everything evenly, this is called "inner balance". Only an imbalance causes a lack of well-being, quarrel, illness, grief and depression.