Could the case of Phineas Gage be explained equally well by Cartesian Dualism and Reductive Physicalism- or perhaps neither can explain it? And if so, what does that suggest about the relation between a philosophical 'theory' designed to make sense of the relation between mind and body, and a 'theory' that's supposed to account for a specific set of events
In his philosophical treatise Meditations on First Philosophy, Rene Descartes proposed that there are two fundamentally different kinds of substances in the universe: the physical, which is extended in space he called “res extensa,” and the mind “res cogitans”. Our bodies are “res extensa” and our minds are “res cogitans”; the two are separate. The reason Descartes theorizes the dichotomy between the mind and the body lies in his philosophy of Dualism. Descartes realized that he could doubt everything except that he was thinking. Since doubting is a form of thinking and thinking requires further thinking, he knew that he must exist, hence the famous syllogism “I think therefore I am.” Descartes further philosophizes that could he doubt the existence of his body and all things physical, but because of his cogito, he cannot deny the existence of his mind. To support his argument, Descartes employs the use of Leibniz’s Law (also known as the Identity of Indiscernibles). Leibniz’s law is an ontological principle which holds that there cannot be separate objects if they have things in common; for instance, if you have two objects and they have all the same property, then you do not have two different objects, you only have one entity. For Descartes, the existence of a physical body can be doubted but the existence of the mind cannot be doubt so they must have different properties—meaning they are two separate objects. However, Descartes’ Dualism leaves a major question: Is the individual the same if an event were to occur that changed his or her brain, shown through the case of Phineas Gage? Phineas Gage was working as a foreman of his crew until, while working with explosives, he experienced a workplace accident in which an iron rod penetrated his left cheek. Miraculously, he survived, but he was not the same man. Following the accident, Phineas Gage’s personality radically changed from being a hardworking and pleasant man to an aggressive alcoholic who is unable to find a job. When seeking an explanation for this philosophical question, the answer lies not in Cartesian Dualism, but through the explanation of Reductive Physicalism. Reductive Physicalism is the view that “everything in the world is made only of physical things.” By this logic, every living being is explained in terms of what encompasses their bodies, such as brains, neurotransmitters, and hormones. Thus, the philosophical theory of Reductive Physicalism is designed to make sense of the relation between the brain and mind.
If Cartesian Dualism were to be right, it raises many questions: For instance, why do people’s minds change depending on what happens to their brains? Descartes had established that the mind and body were separate entities—the mind as a non-physical entity and the body as a physical entity; however, if that were the case, how does a non-physical mind affect a physical brain? For instance, the case of Phineas Gage: Why did an accident bring about a radical change in his personality? The explanation for Gage’s change lies not in Cartesian Dualism, but reductive physicalism. If everything about Gage and his personality prior and after the accident could be explained in terms of his brain, it is no surprise that an accident would bring about a radical change in his personality. For instance, the same could be said to a psychiatrist prescribing antidepressants—like SSRIs—to a patient suffering from depression. In the field of psychiatry, depression is thought to be the result of a “chemical imbalance” in the brain.  For this reason, antidepressants affect certain brain circuits and neurotransmitters, including serotonin; by maintaining levels of neurotransmitters, it could improve communication between nerve cells and strengthen circuits in the brain which regulate mood. Our physical bodies affect our minds.
Although Cartesian Dualism poses many questions, it holds many premises that are arguably irrefutable. One argument is that the mental and physical entities encompass different properties. The mind holds subjective qualities such as what does it feel like or what does it look like. Sensations like these cannot be reduced down to the physical. The weakness of reductive physicalism lies in its belief that consciousness can be deduced from the physical truth. Because of the brain’s complexity, it is difficult to draw and decipher conclusions from people’s mental states.
Cartesian Dualism relies on Leibniz’s law, which as previously stated is an ontological principle that there cannot be separate objects if they have things in common. However, Leibniz’s law can be counterargued by the Masked-Man Fallacy. The masked-man fallacy is explained through the example of Marvel’s superhero Spiderman. If someone claimed that Peter Parker is Spiderman, and since the citizens of New York know that Spiderman saved their city, then the citizens of New York know that Peter Parker saved their city. In essence, what one thinks about the object is not the property of the object itself. If Descartes had established that the mind and body were separate entities, it would not mean that they are completely different. It possibly mean that the mind is part of the physical world.
Despite the flaw of Reductive Physicalism from realizing that the mind holds subjective qualities, Cartesian Dualism fails to explain causal relations between mental and physical events. Another component of Cartesian Dualism is interactionist dualism. Interactionist dualism argues that there are causal relations between mental and physical events. Yet, one of the problems of dualism is how to explain this relation. Philosopher Jaegwon Kim asks, “Can we make sense of the idea that an immaterial soul can be in causal commerce with a material body, and that my immaterial mind can causally influence the physicochemical processes going on in my material brain?” Cartesian Dualism fails to explain the radical change in Phineas Gage’s personality. How could a non-physical entity be affected by a physical object even though two entities—the mind and the body—are separate? However, Cartesian dualists have argued that the brain serves as an antenna and each part tunes in to the consciousness. In this way, brain damage, as in the case of Phineas Gage, could be explained from a dualist perspective. This counteracts the Physicalist’s claim that particular damage caused one to lose consciousness.
Although Cartesian Dualism fails to explain the relationship between mental and physical events, Reductive Physicalism has a major weakness. The weakness of Reductive Physicalism lies from what is missing — Qualia. Qualia are defined as instances of first-person, subjective experience. Australian philosopher Frank Jackson devised a thought experiment known as Mary’s Room as an argument against Reductive Physicalism. The thought experiment goes as follows:
Mary is a neurophysiologist who is forced to investigate the world from a black and white room via a black and white television monitor. Mary learns everything there is to learn about optics, light, the physics of color, and how it affects sensory organs. When Mary walks out of the room and sees color, has Mary learned something new?
To Jackson, qualitative, first-hand experience of seeing color—for instance, the color red—is not the same as knowing objective facts. Jackson explains, “It seems just obvious that she will learn something about the world and our visual experience of it. But then is it inescapable that her previous knowledge was incomplete. But she had all the physical information. Ergo there is more to have than that, and Physicalism is false.” However, there are several objections to Mary’s Room. Firstly, confining Mary to a monochromatic environment would not have prevented her from seeing different colors. Secondly, an individual who is confined to the monochromatic environment, but knows everything physical there is to know, may be able to figure out what colors look like and possibly imagine the experience of seeing the color.
The problem with Cartesian dualism is its “metaphysical claim about the non-physicality, substantiality, and causal profile of the mind.”  Cartesian dualism is not a psychological and scientific account of the underlying nature of the mind. Hence, Cartesian dualism fails to address the explanation for the radical change in Phineas Gage’s personality following his accident. An objection to Cartesian dualism is that the physical world is a “closed system.”  In essence, every physical event is a function that can be predicted and explained in terms of physical events. For instance, with modern medicine, it could be predicted that if a foreign object were to penetrate the human brain, it would cause physiological changes that would impact an individual’s behavior as seen in the case of Phineas Gage.
Aside from the metaphysical claim of the mind presented by Cartesian Dualism, another objection to Cartesian Dualism is its philosophy of mind-body interactionism. Descartes’s philosophy of Interactionism holds that the pineal gland is “pushed” by the activity of the mind. But, it leaves many questions. In Princess Elisabeth’s argument against Descartes, Elisabeth argued that in Cartesian Dualism, the mind is immaterial, thus it has no spatial properties, therefore the mind cannot move the body. However, there is a counterargument to the premise that we cannot conceive of a non-physical “thing.”  Our self-conception encompasses a form of dualism. The mind-body interaction is intelligible because it draws on our notion of embodiment. However, it further leaves us with a question: Which is the primitive notion—embodiment or mind-body causality? If mind-body causality involved the Cartesian notion of an immaterial mind, we could not rationalize the “sense of the ‘mineness’ in casual terms,” meaning how my mind causes changes in my body.  The question of how the mind could cause changes in the body is evident in the case of Phineas Gage. The aftermath of the accident caused behavioral changes of Phineas Gage exemplified through his temperament. Hence, it could be argued that the mind and the body are connected; if an event were to occur in the brain, it would influence changes in a person’s mind.
To conclude, when seeking an explanation to the philosophical question of whether an individual is the same if an event were to change his or her brain, exemplified through the case of Phineas Gage, an answer lies not in Cartesian Dualism, but Reductive Physicalism. Reductive Physicalism holds the notion that much of what encompasses the physical world can be reduced to its fundamental physical basis. Cartesian Dualism fails to explain the make sense of the relation between the mind and brain. Cartesian Dualism holds the belief that mind and brain are separate entities; however, Cartesian Dualism leaves major questions, especially concerning the radical change in Phineas Gage’s behavior following his accident.