Dualism as a Force of Separation Based on a Comparison of the Philosophies of Descartes and Heidegger
Early in the Twentieth Century, Martin Heidegger analyzed the concepts of previous Western philosophers and rejected their prevailing dualistic worldview. Based on Descartes and similar philosophers, Western civilization had divided the world into the dualistic view that everything was either mind or body. In other words, for centuries, they believed that knowledge had two bases: either thoughts or physicality. This philosophy was called Cartesian Dualism.
Since Cartesian Dualism divided the world into mental things and physical things,  it separated mind from body, soul from matter, and spirit from flesh as if they functioned totally separately from each other. It permitted our society to develop concepts based on simplistic assumptions such as all or none, good or evil, and us or them. In addition, that dualism led to a situation where people could be separated from their experiences in the world.
Incidentally, don’t confuse the dualism of Western civilization with the apparent dualism of Taoism that is commonly expressed by the yin yang symbol. The yin yang of Taoism incorporates an interdependent and independent existence of opposites. The counterparts may be divided into dark and light, male and female, hot and cold, or moon and sun. However, just as the moon and the sun exist in relationship to each other, yin and yang interact. They seem to stand alone but there is that little dot in each that symbolizes interaction and mutual support. Together they make a complete balanced whole. The yin yang concept does not completely separate the opposites; even though they are independent, the Taoist symbol, and the philosophy behind it, does not detach the mind from the body or yin from yang.
Working from a purely Western European perspective, Heidegger rejected the dualistic worldview. He believed that philosophy must focus on human experiences in the world, and so, he developed some complex and novel explanations to try to explain events. Heidegger said that people could not be separated from their experiences. In fact, he viewed experience as an important part of existence, but he recognized that there is no analysis of event or things while experiencing them.
For instance, individuals using a hammer do not think about the hammer unless it breaks. That is, to use the hammer, they do not need to think about how many nails it has pounded into wood in the past, and they do not need to know how or where the hammer was manufactured. They are focused on utilizing the hammer, perhaps to build a bookcase. Explanations about how the hammer works are not an important part of the experience. The company that manufactured it is not important unless the hammer breaks and must be replaced.
In the same way, Heidegger considered Being-in-the-world to be an experience. To him, a mental analysis of any object was not considered important; only the usefulness of the object was important. Yet, Being-in-the-world was imperative. That is, the essential thing about life would be actually living our experiences.
It is important that we don’t separate mind from body or spirit from flesh. But the key is: actually living our lives is of utmost significance. I think we all can agree with that.
— Footnotes —
 Martin Heidegger (1889-1976) was a German philosopher; his most famous work is Being and Time.
 René Descartes (1596-1650) was a French philosopher, scientist and mathematician; he was (and still is) extremely influential in western philosophy and analytical geometry.
 These explanations of Heidegger’s views are mine; any misinterpretations cannot be blamed on the reference material.
 I am grateful to Professor John Searle, University of California at Berkley for his succinct online lecture explaining Cartesian Dualism.