The “mind,” complete with personality, memory, emotion, sensation, volition, and desire, is one of the most distinctive properties of the human person. The origin and explanation of the mind — where it comes from and what accounts for its existence — has been a topic of human curiosity since we arose from lower lifeforms.

The first explanations of the mind were supernatural in nature: a god or gods gave us a “soul” or “spirit,” some ethereal substance, separate and distinct from one’s body, which makes up the mind or consciousness. This view, known as dualism, is popular in many animistic belief systems, as well as in major world religions like Islam, Christianity, Judaism, and Hinduism. These religious traditions assert that this “soul” survives a person’s physical death, allowing for one’s existence to continue indefinitely.

The earliest philosophical inquiries into the subject were also dualistic in nature. René Descartes, a 17th-century French philosopher best known for his Pensées and “Cogito ergo sum” principle, formulated a position known as substance dualism, sometimes called interactionist dualism, which posits that an immaterial mind (or “soul”) somehow causally interacts with a person’s physical brain. Descartes’ position was largely based on Christian doctrine, and it faces a number of scientific and logical problems (more on this later . . . ).

While it was the prevailing view in the philosophy of mind during the 17th century, substance dualism fell out of favor during the 19th and 20th centuries, being replaced by various forms of physicalism, a view which posits that the mind is reducible to physical, natural activity within the brain. Physicalism, which is the prevailing view in contemporary philosophy, comes in many different forms, such as property dualism, epiphenomenalism, identity theory, and eliminative materialism. Although there are important distinctions between these kinds of physicalism, this post will focus on showing why physicalism as a whole is preferable to interactionist dualism as an explanation for the human mind.

Arguments for Physicalism

Evidence from modern neurology offers a compelling case to accept the physicalist account of human consciousness. One such channel of evidence is the fact that a clear, consistent linkage exists between certain areas of the brain and corresponding states of mind.

For example, scientists have discovered that Broca’s area and Wernicke’s area are directly linked to speech production, and have demonstrated that patients with injuries to these parts of the brain demonstrate specific impairments of speech and communication (such as, for example, Wernicke’s aphasia). Researchers have discovered similar direct linkages between the brain and other mental “areas,” including mathematical ability, sleep, love, sexual desire, and short-term memory. These clear linkages between brain areas and mental activity would be unexpected on interactionist dualism, but are perfectly consistent with physicalism.

Perhaps even stronger evidence for mind-body physicalism is the effects of brain damage on the most fundamental aspects of a person’s identity. For instance, anyone familiar with the horrific symptoms of Alzheimer’s disease, a type of dementia, can attest to the changes in personality, identity, and memory sufferers undergo as the disease progresses. Individuals who were once devout, kind, and respectful can irritable and aggressive due to the disease, while extroverted, gregarious people can become withdrawn and sullen. 

People suffering from Pick’s disease, a form of frontotemporal dementia, can even undergo complete changes in their political views, going from socialist to conservative (or vice versa), or even their religion, going from theist to atheist (or vice versa). Dramatic changes in personality can also occur due to acute brain damage. The case of Phineas Gage, a 19th-century industrial worker, illustrates this: a rod thrust through his brain due to a work accident changed him from a socially conservative, moral, mild-mannered man into a wild libertine with little regard for social convention.

This scientific evidence is very problematic for substance dualism, but is perfectly explained by a physicalist account of the mind-body problem.

Arguments Against Substance Dualism 

In addition to the scientific evidence for physicalism, there are strong philosophical arguments against substance dualism. One of the strongest of these is the argument from location. This argument starts from the principle that location is a physical property, and that, on substance dualism, the mind is a nonphysical entity. By definition, a nonphysical entity can’t possess a physical property (otherwise it would be a physical entity), so it follows that, on dualism, the mind can’t be located anywhere, and therefore cannot interact with a person’s brain, which is a physical object with a physical location.

A second problem facing interactionist dualism is Occam’s razor, a philosophical principle which states that the simplest explanation of the evidence is likely to be correct. Physicalism offers a simpler explanation than substance dualism, as it posits no extraneous entities (like a soul) to explain the human mind.

Another logical hurdle substance dualism trips over is the problem of interaction. Stated simply, the problem of interaction raises the question of  how two fundamentally different things (physical and nonphysical), with no properties in common, can causally interact. For X to exert a causal force on Y, X must have at some properties in common with Y. However, a nonphysical soul and a physical brain have no properties in common, so it is incoherent to suggest that one can impact the other.

Conclusion

The evidence from science and philosophy clearly shows that substance dualism is at best highly unlikely, and at worst impossibly incoherent. A physicalist account of consciousness provides a parsimonious explanation for the evidence while not contradicting any part of observed reality. While I don’t fault those who hold substance dualist positions for religious reasons, I don’t feel the dualist position can be rationally defended.

Side note: if you want a less technical, more “fun” explanation of why substance dualism fails, check out this video!:  

Recommended Readings

If you’d like to read more on this subject from someone far more qualified than me, check out:

Popular-level books:

Tufts University philosopher of mind Prof. Daniel Dennett’s Darwin’s Dangerous Idea

Duke University philosopher of science Alex Rosenberg’s The Atheist’s Guide to Reality

For those interested in a more technical, academic articulation:

University of Maryland philosopher Peter Carruthers’ chapter The Case for Physicalism