Typical and even archetypal is the dualism of Descartes. The mind is composed of a mental substance, and the body is composed of a physical substance. The key feature, according to Descartes, of physical substance is that it is extended spatially. The key feature of mental substance is that it thinks, or at least that it can think.
Dualisms like this cartesian paradigm have both strengths and weaknesses, which are revealed as dualists debate with monists.
Monism asserts that there is only one level of reality. This simplified ontology, while in some ways unsatisfying, relieves its proponents of the task of explaining some type of interaction between two different types of substance.
Looking at monisms, scholars often distinguish between the materialists and the idealists. Materialists would be those who assert that a physical level of reality exists, and is indeed the only level of reality that exists. The materialists argue that there is no such thing as a mental substance. Most forms of materialism assert, among other things, that “humans are entirely physical beings,” in the words of Scott Calef.
Idealists, on the other hand, are monists who deny the reality or existence of a physical substance, and argue that only mental substances exist. As Paul Guyer and Rolf-Peter Horstmann write, the “ontological” or “metaphysical” form of idealism asserts that “something mental (the mind, spirit, reason, will) is the ultimate foundation of all reality, or even exhaustive of reality.”
One way to phrase it is this: the materialist asserts that only physical substances and objects exist; what seems to be a non-physical object, like a mind or a memory or an idea, has no real independent being. The idealist, by contrast, argues that there are no independently really existing physical substances or objects, and what seem to be objects, like rocks or rivers or trees, are merely ideas or constructs in our non-physical minds.
Over the centuries, a number of major philosophers have lined up on either side of this debate, and each has produced a unique and nuanced version of one side or the other.
Thus far is a more-or-less standard account of dualism and monism, and of the two types of monism.
The question now at hand is this: Is it possible or plausible to argue that the two monisms, idealism and materialism, are reducible to each other? Because each asserts that there is only one level of reality, only one type of substance, can it be that it makes no sense to label this one substance as either ‘material’ or ‘mental’?
Do the words ‘mental’ and ‘physical’ have usefulness only in the context of two types of substance? If there is only one type of substance, to label it as ‘non-physical’ or ‘physical’ might be senseless.
The materialist argues that the mind is the physical brain and its neurochemical and neuroelectrical processes; the materialist argues that there is no non-physical mental substance which composes the mind.
The idealist argues that the mind is composed only of a non-physical mental substance, and that there is no physical substance which composes the brain.
Aren't the two of them saying the same thing? Both of them agree that mind/brain dualism is wrong. Both are saying that the mind exists in one level of reality, composed of one type of substance, and that there is no other type of substance. One might appeal here to Leibniz and his identity of indiscernibles.
Assuming that dualism were ruled out altogether, and that either materialism or idealism must be an accurate description of reality, which experiment would one conduct to decide between the two? If monism is granted, then is there any detectable or verifiable difference between idealism or materialism? Or are all monisms essentially one?
