Source: p 33, What Does It All Mean? A Very Short Introduction to Philosophy (1987) by Prof. Thomas Nagel
But to discover that tasting chocolate was really just a brain process, we would have to analyze something mental -- not an externally observed physical substance but an inner taste sensation -- in terms of parts that are physical. [1.] And there is no way that a large number of physical events in the brain, however complicated, could be the parts out of which a taste sensation was composed. A physical whole can be analyzed into smaller physical parts, but a mental process can't be. Physical parts just can't add up to a mental whole.
A novice in philosophy, I cannot imagine or conjecture how physical parts CAN constitute a mental whole. But I must not appeal to ignorance and assert that it CANNOT.
So why does Thomas Nagel opine the 3 sentences after 1 so decisively?