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Dualism is the metaphysical doctrine that there are two substances, i.e., distinct and independent types of being, one material and the other spiritual. Material substance is defined as physical 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 and technical instruments that extend the range of the senses, such as electron microscopes, telescopes, radar, etc.
The spiritual world is usually 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 is 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, we can't infer 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 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 these psychological or mental activities are best explained as functions of a non-physical substance. They can certainly be coherently explained by dualism, but it is not necessary to bring in the belief in non-physical reality to explain everything that is hard to talk about physically. As noted above, there are also problems with trying to conceive of non-physical perception, despite Descartes' argument that such a conception is absolutely clear and distinct in his Meditations on First Philosophy, an argument essential to dualism.