Dualism is the belief that there are separate physical and non-physical aspects of reality. This leads to the famous mind-body problem: if part of us is physical, and part non-physical, how do they interact? "Where" are they connected?
I don't claim to have the answer to that (although I do have a thought or two about it), but think it's still better than the other main alternatives: Idealism and Materialism/Physicalism.So, I believe that if one is both a scientist and believes in any kind of spirituality, one is actually a dualist.
For an entertaining general description of philosophy, see
The Big Dummy's Guide to Theology, Philosophy, and Ethics.
Sept. 2002: This link finally works again; I found its new home!