'Physical parts just can't add up to a mental whole.' : Why does Thomas Nagel assert this impossible?

Question

_{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?

Answer 1

The key is in the nuances of the word "add up" and the world "analyze". Nagel is not here saying that all of the physical parts together cannot constitute a mental experience but that however this works, it cannot be merely additive.

Nagel is asserting that if we're looking at say "tasting chocolate," there's no half of the brain that does half of the tasting and another half the other half of the tasting. Instead, for it to be tasting, everything has to work together or there's not tasting at all.

Thus, the physical bits don't "add up" because the mental is not the sort of thing that's gotten to by merely adding up a whole bunch of other things. Maybe to give an analogy might help. Putting one leg ahead of the other rapidly means that I'm running, and I'm still running even if you shorten the length of my run by several footfalls. Conversely, if I'm writing a sentence in English: "he walks the dog", the sentence is no longer grammatical if you take a single word out of it. What remains is something else entirely. This is the essence of the "add up" claim.

Conversely, for physical things, we can (until we hit the limits of physics or our apparatus) analyze them into smaller physical processes. Thus, they are like "running" above. A mental state on the other hand seems like the sentence above -- at least for Nagel -- meaning it doesn't work if you take any piece away from it. (I would wager there are several big philosophers who'd disagree with him, but that's kind of what it means to have a position in these debates).