Mental states are intrisically knowable, but physical states require inference and experience.
The dualist position hinges on the idea that human beings are made up of two components; the physical and the non-physical. Our body and brain, everything we can touch, move, feel, and manipulate, lies within our physical construct. But we also have a non-physical mind. Our consciousness, emotions, desires, and experiences occupy the mind. They cannot be observed, touched, or perceived, but they still exist in the non-physical realm. While there are many different brands of dualism, the core idea of “dual” entities- a physical and a non-physical- connects them all. Plato was one of dualism’s earliest proponents. In his Phaedo, Plato promoted the idea that humans had eternal “Forms” which provided intellect and gave us the capacity for understanding. He suggested that our immaterial Forms were imprisoned within our physical bodies
When I decide that I need a glass of water. I do not need to make an inference or an assumption based on evidence. I don’t need to calculate the time since my last glass of water to know I need another one, nor do I have to feel my tongue and deduce if it is dry. I know I need a glass of water intrinsically. Mental states are knowable without inference or experience, unlike physical states, which can be deceptive and prompt error.
Neurological Correlation Critics of dualism have pointed to neurology and neuroscience as evidence that dualism is misplaced in its approach. When we have a mental thought, for example, imagining myself walking through the front door to my home, a certain part of the brain which correlates to special recognition becomes active. We can clearly see this in MRI scans. Even something we know intrinsically, like needing a glass of water, has corresponding neurological activity. Similarly, when I experience pain, there is c-fibre stimulation inside the brain. This suggests that there is a physical process behind each thought and that rather than mental states occupying the realm of the non-physical, they are nothing more than a collection of neurons firing in the brain.
Some mental states I know to be true without inference. Therefore these are innate and not experienced. Therefore these must be non-physical. Therefore, humans must be made up of physical and non-physical substances.
Our mental states correspond to physical activity in the brain. Therefore, our mental states are not non-physical, they are physical. Therefore, humans are not made of two substances. They are only made of physical matter.