To The Skeptic's Dictionary - Table of Contents
The metaphysical doctrine that there are two substances, i.e., distinct and independent types of being: material and spiritual. Material substance is variously defined as physical or material and is asserted to be the underlying reality of the empirical world, i.e., the world we see, hear, etc., and measure with our senses as well as with our technical instruments such as electron microscopes, telescopes, radar, etc..
The spiritual world is variously described negatively as the non-physical, non-material reality underlying the non-empirical world, variously called the psychological, the mental or the spiritual world.
Dualists are fond of a belief in immortality. If there is another type of reality besides the body, this non-body can survive death. The non-body can conceivably exist eternally in a non-physical world, enjoying non-physical pleasures or pains distributed by a non-physical God. This notion seems to me to be non-sense, but it apparently gives many people great comfort and hope.
Some dualists are fond of drawing a significant inference from the fact that we use different kinds of language to talk about physical things and non-physical things. They note that when we talk about physical things we use language that locates or causally connects objects in space. When we talk about processes such as thinking, however, we don't use the language of things in space. We don't think of thinking as taking place in a particular place or of a thought as having physical dimensions. That is true; however, the dualist infers from this fact about language that the non-physical is a substance, i.e., a type of reality capable of independent existence, not reducible to some other phenomenon. Most dualists would agree that colors, for example, are not substances because colors do not have an independent existence: they are reducible to other phenomena, such as light, sensory apparatus, etc.. Yet, many dualists would deny that thinking, perceiving, willing, desiring, etc. are reducible to material processes (e.g., brain states). They believe that these psychological or mental activities are best explained as functions of a non-physical substance. They can certainly be coherently explained by dualism, but I don't believe it is necessary to bring in the belief in non-physical reality to explain everything that is hard to talk about as a thing in space.
In any case, it seems very presumptuous to assume that the way, language has evolved and developed is a keen sign as to the nature of reality.
See entries for astral projection, mind, near-death experiences, and souls.
further reading
Ryle, Gilbert. The Concept of Mind (New-York: Barnes and Noble: 1949).