On page 154 Deutsch ascribes to Dennett a perspective which I hold. Deutsch describes the perspective better than I have and better than I recall Dennett having described it:
Dennett’s perspective is comfortable to me because I have programmed computers where almost all program state was continuously in memory and calculation was indeed inspecting memory.
Dennett and Deutsch both talk as if they disagree on some fact of the matter—something to be decided in the lab. This argument seems preposterous to me. Deutsch seems to be an implacable realist and insist that there is one true perspective. I don’t recall whether Dennett was also inflexible.
The explanatory power of perspectives is not necessarily ordered. Some perspectives explain some observations and others explain others. We were stuck thru the 20th century on the QM-GR split and I bet we will be stuck for much of the 21st century too. Within QM there is the wave particle duality and only vague hand-wavy attempts to argue that they don’t ultimately conflict. There is the group-selection vs. selfish-gene conflict, but I am unconvinced they lead to different predictions. Sometimes it is easier to explain something in one coordinate system than another. You must worry when two perspectives predict different observations.
In this regard I like Deutsch’s emphasis on explanatory power. It fits with Feynman’s dictum.
Deutsch: “If you can’t program it, you haven’t understood it.” Bravo. But even if you can program it the reality of some of the constructs is still up for grabs. Even programs have emergent properties and can be accurately described from different perspectives. If by memory you mean RAM, then the logic of the program does not depend on whether some interrupt flushes some cache lines to RAM. An ‘explanation’ of the program will not fit in one’s head if he must wonder about that contingency.
I find Deutch’s ideas about proofs amusing and possibly useful. I first claim that the notion of mathematical proof is something to be defined by mathematicians and that they choose to define it in the Platonic realm. Proofs are like numbers and are not physical. A mathematician finding a proof is physical, just as a mathematician finding some number with a given property is physical; the proof is Platonic. His speculations on proofs with an infinite number of steps is interesting in the same (important) sense as Cantor’s transfinite numbers are interesting. People have considered such possibilities and no inconsistencies have appeared as far as I know.
On page 202 Deutsch quotes Hawking:
On page 263:
For now I do not buy Deutsch’s notion of fungibility as elaborated at the end of page 268.
I cannot help but compare the last chapter with the last chapter of Tippler and Barrow’s “The Anthropic Principle”. Deutsch himself makes the connection. These chapters speculate on the ultimate destiny of intelligence in the universe. They are both wildly optimistic and that makes them both fun to read.