Before we begin, I'd advise checking out “Metaphysics 101: The Importance of One's Ontology”. I plan on picking up where I left off. The aim in this primer is to make explicit the complications that can arise by adhering to a basic metaphysical principle. We'll start by examining the principle below and then demonstrating how a specific philosophical puzzle can help upend an entire worldview: Things exists that are not material (physical) in nature.
It's easy enough to see why this can be an attractive viewpoint, as immaterial objects are impossible to observe (by definition) through physical means. There is no test you can create whereby you could prove the existence of an immaterial object using material means. This is due to the fact that immaterial objects by definition cannot have physical properties. If they did, they would fall back under the umbrella of materialism, destroying the distinction. As such, belief in immaterial objects allows you to posit things that do not lend themselves to physical descriptions. This is important in religion, as many of the key features of religion (immortal souls, spirits, deities, etc.) are supernatural and require a non-physical existence. A material god, bound by the rules of physics, would be substantially different than an immaterial god wholly unbound by physical restraints.
Religious entities are not, however, the only type of immaterial objects. Some have posited that numbers are real yet immaterial objects. Plato famously held this view and he was far from the last to do so. 'Universals', or properties that are shared by numerous objects, are another contender for real, immaterial things. To briefly illustrate his view on this, Plato believed that objects were red insofar as they participated in the universal 'RED'. RED existed in the realm of the Forms, an immaterial heaven that ultimately grounded physical reality. Interestingly, Plato thought that these immaterial objects were what truly existed while the physical world was a mere shadow of their metaphysical brilliance (needless to say, this is no longer a mainstream position). Others, such as David Lewis (a 20th century philosopher) believed 'relations' and 'properties' existed as surely as the four forces of particle physics, even though 'relations' and 'properties' are definitively immaterial. I do not wish to get sidetracked by the examples listed above, but I do aim to point out that immaterial objects are not exclusively religious/spiritual in nature.
The fact that there are some somewhat uncontroversial conceptions of immaterial objects is not to say that belief in those objects cannot be problematic. For an example, the immaterial/material distinction has been hotly contested within theory of mind, the branch of philosophy that focuses on the nature and importance of consciousness, mental events, mental functions, and the like. It aims to describe what minds are, their causal role (if any) and the mechanisms by which minds and bodies interact. The discussion surrounding this relationship between an immaterial mind (or soul, under some definitions) and the physical brain/body has been labeled the 'mind-body problem'. This problem puts forward the following question: what is the relationship between the non-physical mind and the physical brain? Fundamentally, how does an immaterial object interact with a material one?
One of the first to crystallize the idea that the mind and body are categorically distinct in a philosophical context was Rene Descartes, the French philosopher and mathematician. His specific conception of the mind and its interaction with the body is called 'mind-body dualism' and it states that the body and mind are substantively different. This idea is also called 'substantive dualism' for that reason. Descartes aimed to show that the mind was ultimately the source of all conscious action and he put forward some clever philosophical arguments that lent credence to the notion that mind was more fundamental than body. He could not, however, describe how this non-physical entity interacted with its physical embodiment. making this a great example that illustrates the potential consequences seemingly innocuous assumptions can have. Let's delve a little more deeply into the subject.
The Enlightenment - the time period Descartes lived in - gave rise to the idea that most phenomena (if not all) could be explained in purely physical terminology, without appeal to abstract and immaterial concepts that had often dominated earlier intellectual thought. This contrasted with the prior worldview that interpreted those same phenomena as events laden with intention. We'll examine a classical example in Aristotle's philosophy to demonstrate this mode of thought. Aristotle believed in what is called a teleological worldview. Teleology explains action in terms of goal-orientation.
So, why does the rock roll down the hill under a teleological worldview? The rock does so because it aims to find the lowest point at which to rest, which happens to be at the bottom of the hill. 'Finding the lowest resting point' is the teleological goal of the rock, and so it rolls down hills. For another example, it was normal to analyze natural events, such as plagues, terrible storms, earthquakes (etc.) through a metaphorical lens. Plagues were punishments doled out by an angry God who demanded righteous behavior from his plague-stricken victims. Prior to the Enlightenment period the natural world was, in short, one loaded with intentional action and metaphorical meaning. Events happened not because of physical necessity but because of an altogether different type of cause and effect:
Premise (1) Bad things happen to those who are wicked.
Premise (2) Bad things are happening to us.
Premise (3) Since (1) must logically follow if (2) is true, we are wicked.
Aristotle heavily influenced medieval theology, so his philosophy was often cited by the Catholic church when developing its own theology and dogma (which, de facto, became the metaphysical worldview of Western civilization). This type of interpretation was dominant in the Catholic church into the Renaissance, which means Western thought was heavily influenced by these ideas for well over a thousand years.
To return to our earlier discussion, the mind-body problem became apparent as Descartes was attempting to explain how the physical body interacted with the immaterial soul. As the soul 'makes the man', this interaction was crucial to his worldview, but he could not offer an explanation how these two objects interacted. For Descartes, the soul and mind were synonymous. This Cartesian soul mind is what animates the body and is the seat of conscious, intentional action. When someone is hungry, the soul/mind is what tells the body to get up and get a tasty snack, but Descartes (and those who adhere to substance dualism) could not philosophically bridge the physical/immaterial divide. The old intentional, metaphorical worldview was clashing with the new physical, rule oriented methodologies of the scientific enterprise. The issue is that an immaterial object cannot have physical properties, as envisioned by Descartes. How is it that the Cartesian soul can cause neurons to fire? How can it make muscles contract? These things happen due to interaction of material objects with physical properties, but an immaterial soul has none of these features. How, then, does it interject itself into a causal relationship with the body?
There have been many proposed solutions to the mind-body problem, but I'm not interested in going into those at this time. What I have highlighted are the complications that can occur based on what sort of objects one admits into one's ontology and the unintended consequences utilizing these objects can have within one's metaphysics. Based on a switch to a more scientific interpretation of events the older intentional worldview was no longer explanatorily satisfactory, undermining a metaphysical outlook that had dominated Western thought for a thousand years. The 'mind-body problem' is one example of how an entire metaphysical worldview can be unhinged by a specific philosophical problem within one's ontology. There are many consequences associated with such a basic choice as allowing for immaterial objects, and these often go undetected if we do not take the time to analyze them. Believing something as natural as the mind is immaterial is not a free lunch. Serious philosophical problems are waiting for you once a commitment to an proposition like this are made. Reflection on the magnitude of those consequences and the fallout to which we may then be committed is an important step in developing a coherent, consistent metaphysics. At the end of those reflections we sometimes realize that the payoff does not justify the costs we are later made to pay to justify or defend our earlier assumptions.
Featured image of Rene Descartes by Frans Hals, 1649 CE.