What is dualism? If I were to define it, I would say it is the separation of specifically two things; as if one is superior and the other is inferior. Generally speaking, Western Philosophy traces dualism back to Socrates; and to this day has not really let up. Throughout Socrates life, he generally said that he longed to experience the mind without the body. Even on his deathbed, he told his friends that a philosopher should be happy to experience death; because it is the time when the physical body dies and the mind alone is able to exist without senses. To further the definition I gave at the beginning of this paragraph to entail the "traditional" definition of dualism, dualism is generally considered the separation of the physical and mental properties.
In many similar ways during a similar time period, Asian philosophies were attempting to do the opposite; bring the body and mind into unison with one another. Yoga is perhaps the most popular form of doing this, though many individuals (especially in the Western world) are unaware of yoga beyond the physical exercise experience that is yoga. The United States is so caught up in the physical and material realm that yoga is sold as an educational physical class. Typically, people seemingly take yoga just to "look better", completely overlooking the entire concept of yoga altogether.
Back on topic, the reason I really am interested in discussing dualism is because I feel dualism is unavoidable in many popular philosophies. Even at the end of a yogi's (one who does yoga) journey, there is a separation of the body and mind in an extremely similar way to that of Socrates and Descartes. If one was to peak into really any Western church service, declarations of a dualistic way of thinking would undoubtedly be found. Many of us growing up with a notion that we are more than just physical bodies, for better and for worse.
So frequently do some philosophers attempt to teach people to "break down the walls" of dualism. The largest problem with this idea is that by creating and ensuing a world without duality, a new dualism is created: the dualists vs. the non-dualists. While this is not the same sort of existential dualism as often thought about, it is an important note to consider philosophically. In the least, some dualisms (or...opposites) exist. The question is whether or not their is an opposite of the material world. We can make logically arguments on which we presume that their must be an opposite, simply because most things in the world do have opposites; however that in itself is not evidence for a duality.
Dualism has reached a new level of discussion, in such a way that religion and philosophy as a whole has reached a new level. Through college and the internet, the prominence of many beliefs can be found with little effort if you are willing to look. However, the discussion is no longer a peaceful chat between friends, but ultimately now leads to war and death.
How so? Examine all wars ever fought in the history of the world in the name of a higher power, justified in some way. Are not these wars about who is more superior and likewise who is inferior? Who is enlightened and unenlightened (Plato's "Allegory of the Cave")? The infamous Crusades were fundamentally waged due to many individuals believing they are right, and the other side is wrong.