You can ask how a burnt finger feels, but not how glutamate feels
It makes sense to ask what a blue sky looks like or what a burnt finger feels like. Asking what a surge of glutamate in the prefrontal cortex feels like sounds odd, even meaningless. That mismatch sits at the core of the mind-body problem: how can electrochemical tissue produce experience at all?
Philosophers call the felt, subjective side of experience qualia. David Chalmers argues that someone could know every objective fact about seeing red, from brain states to light wavelengths, and still not know what seeing red is like. The phrase what it is like was coined by Timothy Sprigge and popularised by Thomas Nagel. An older puzzle, traced to John Locke's essay of 1690, imagines an inverted spectrum: your red might be my green, with both of us using the same word.
Two broad camps answer the problem. Dualists hold that the mental is in some way non-physical. Plato already separated intelligence from the body, but René Descartes, writing in 1641, gave the classic version: mind is a thinking, non-extended substance distinct from the brain. He was the first to frame the problem in its modern form. Property dualists instead see mental properties arising from the brain without being reducible to it. Something similar can be found in the ancient Indian Samkhya and Yoga schools.
Dualism must explain how an immaterial mind moves a physical body. Arnold Geulincx and Nicolas Malebranche answered that God intervenes every time. C. S. Lewis offered an argument from reason: if every thought is simply the effect of physical causes, we would have no grounds for trusting any conclusion, including monism itself.
Monists reply that there is one kind of stuff. Baruch Spinoza held this in the seventeenth century; idealists say everything is mental, while neutral monists such as William James treat events as mental or physical depending on their relations. Most philosophers today are physicalists of one sort or another, some expecting neuroscience to explain mental states fully and others insisting mental vocabulary cannot be replaced. John Cottingham has even proposed a trialism, adding sensation as a third element.
Source: Philosophy of mind
