Interactionism is the common sense view of mind-body interaction. Interactionists hold that there is two-way causation between the mind and the body, both that mental events cause physical events and that physical events cause mental events.
If I decide to stamp on your foot, and my foot then lands on yours, then it certainly seems that my decision (a mental event) causes the stamping (a physical event). If when my foot lands on yours, you experience pain, then it certainly seems that the stamping (a physical event) causes the pain (a mental event). It thus appears obvious that causation occurs in both directions between the mental and the physical: physical events cause mental events, and vice versa.
The basic difficulty for the dualist arises from his claim that mind and body are utterly different types of substance. Given this difference, it is bound to be difficult to explain how either can effect the other.
The problem, however, can be stated more rigorously. For any physical event, there is a sufficient physical explanation. Electrical events in the brain are causally sufficient to bring about bodily movement; why, then, postulate a second, mental cause of that event?
In fact, postulating a mental cause raises scientific difficulties. The universe is supposed to be a causally closed system. Mysterious mental causation of physical events violates this principle of science that has been so fruitful.
This scientific problem can be avoided if the dualist is willing to reject interactionism and instead adopt epiphenomenalism. Epiphenomenalism holds that mind-body interaction only occurs in one direction: from the physical to the mental. According to epiphenomenalists, physical events give rise to mental events, but not vice versa.