Summary
Highlights
Descartes defines the body as material with the essence of extension, and the mind as an immaterial substance whose essence is thinking. This view requires that mind and body are not just different, but distinct substances.
Descartes' argument in 'Discourse on Method' states: I can doubt my body's existence but not my own, therefore I am not identical to my body. This argument is deemed fallacious, similar to assuming Hesperus and Phosphorus are different because one can doubt their identity. The issue lies in treating 'being doubted by me' as a property of the object itself, rather than a subjective state of knowledge.
Descartes presents a stronger argument: I can clearly understand myself as a thinking, non-extended thing, and a body as an extended, non-thinking thing. If God can create these as separate, they must be genuinely distinct. However, this relies on certain premises and the notion of 'clear understanding'.
Descartes' move from 'I am certain I am a thinking thing' to 'I am only a thinking thing whose essence is to think' is problematic. Knowing one is thinking doesn't mean one's essence is solely thought, as illustrated by John Locke's idea of a thinking stone. This confuses what is known about an object with its inherent nature.
Descartes argues that because God could create mind and body separately, they are distinct. This again runs into the issue of epistemological versus metaphysical possibility. Just because one can imagine Hesperus being destroyed while Phosphorus survives doesn't mean they are distinct objects, especially if they are factually the same.
A significant challenge is how two distinct substances—an immaterial mind and a material body—can interact. Descartes believed causation was intelligible. However, a Humean view of causation, which sees cause and effect as merely constant conjunction, could potentially alleviate this problem by allowing for law-like connections between mental and physical events, even if their mechanisms aren't fully understood.
The principle of causal closure of physics states that physical events are caused only by other physical events. This poses a problem for dualism, as mental events are seen to cause physical actions (e.g., raising an arm). However, the video argues that the causal closure principle is more an ambition of science than a proven fact, and it's difficult to experimentally verify or refute mental causation in a complex system like the human brain.
Objections from evolutionary theory are considered more worrisome for dualists. It's difficult to explain how an immaterial mind could have evolved, and at what point in evolutionary history minds would appear, or if they are a 'separate substance' from the very beginning of life.