First Person Authority
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. The roots of most modern interpretations of dualism come from René Descartes. In Descartes’ Meditations on First Philosophy (1641)  he outlines his position of substance dualism. Descartes concludes that the mind and body come together to make up “one single thing” (1980). He went further than his predecessors and asserted that the two were conjoined, even offering the pineal gland as the location where the physical and non-physical components meet. When we move our body, according to Descartes, the pineal gland impels “the spirits towards the muscles which bring about this effect” (1952, p.299). If humans are only physical entities and do not possess a non-physical aspect, then the laws of physics must govern every aspect of our being. Laws such as Newton’s Law, whereby every action has an equal an opposite reaction, must also apply to our mental states.
This principle would mean our free will is determined by physics. Our morality and ethics also. This means that our free will, desires, ethics, and personal values were created by an action and have an equal and opposite reaction. Our free will would be reduceable to mechanical causation, thereby making it not free, but simply a manifestation of physics and mechanics. If we are to believe we are truly free and have full agency over our desires, then our free will cannot occupy the physical realm; it must exist in a non-physical sphere, free from the laws of physics.
Interactionism If the physical world is bound by the laws of physics, how can something non-physical interact with something physical? If two objects collide, the movement and force of one object will cause the other object to change its speed and direction. This is a physical law of causation. However, if we are to believe, as Descartes argues, that the mind influences the body and causes its movements, how would a non-physical thing interact with something physical? It is not compatible with our understanding of modern physics. Similarly, if the mind were to causally interact with the body, it would require energy to move a physical object. If this was the case, and the non-physical world was interacting with the physical one, science would dictate that the energy levels in the cosmos would be ever increasing as mental energy was constantly being converted to physical energy. But this is not the case.
Our mind and mental states cannot be reduced to a physical thing. Our body is physical. Therefore, humans must be made of two components, a physical body and non-physical mind.
Physics dictates the non-physical world cannot impact the physical world. Therefore, our mind cannot have any influence over our body. Therefore, our physical actions cannot be dictated by the mind.