Dualism refers to the concept that there are two fundamental principles operative in the world and that these two principles are independent and irreducible. The concept of dualism is important within both a philosophical and theological framework.
Philosophically, the question is most often concerned with the relation between mind and body is thus intimately concerned with the question of consciousness. There are two basic forms of dualism, substance dualism and property dualism. Substance dualism holds that the mind is a separate entity, non-physical in nature. Property dualism holds that the mind cannot exist separately from the body or person which is said to have two irreducibly different properties, mental and physical.
Substance dualism is the belief that reality is composed of two fundamentally different types of 'stuff'- material 'stuff', and immaterial 'stuff'. Science is concerned only with the study of material 'stuff', whereas theology and mysticism lays claim to a study of any immaterial 'stuff'.
Substance dualism was at the core of Plato's philosophy. He argued that the constantly changing, imperfect material world was merely an shadowy reflection of the immutable, perfect immaterial realm of the Forms. This immaterial realm could only be accessed by human reason.
Descartes also advanced a specific form of substance dualism known as Cartesian Dualism. He argued that there was only one thing which was immaterial- the human mind. When a person has a thought, such as 'I am hungry', it results in the immaterial mind affecting the brain's pineal gland in such a way that the brain sends electronic signals to simulate various muscles to contract so the body eats. This in turn results in changes in the pineal gland, which make the mind feel less hungry and sated.
However, in the 1960s, Ryle famously argued that substance dualism, particularly Cartesian dualism, was nothing more than 'the ghost in the machine'- a bizarre, ineffectual, unnecessary force that contributed nothing to our understanding. He argued that concepts such as the soul- long held as the classic example of an immaterial thing- result from a specific misuse of language called a category mistake. He further argued that, if there was an immaterial realm, either it would have to be wholly separate from our own material realm (leading to theories of non-interactionism such as epiphenomenalism and parallelism) or that God is deliberately deceiving our senses whenever such interaction occurs, as there is no empirical evidence of dualist interaction in the material world.
This has led many modern theologians, such as Hick, to abandon the ontological position of substance dualism in favor of materialism. This, however, theoretically renders religious hypotheses, such as Hick's replica theory, scientifically testable, and it remains to be seen what results this avenue of thought will produce in the future.