A good guess, if it's someone else than Draco. But where and when did that happen? Are you referring to Harry's comment "I thought of my absolute rejection of death as the natural order." in Chapter 46? Neither of the gentlemen present thought that was sufficient information for understanding how to cast a Patronus.
Unfortunately I can't make it this time either. Have a fun meetup! Maybe the next one could be an informal picnic? We should enjoy the short summer while it lasts :)
All rationalists in Finland, FYI: http://www.kryoniikka.fi/liity-jaeseneksi
Maybe worth noting that there's recommended reading on decision theory on the "Best textbooks on every subject" post.
On decision theory, lukeprog recommends Peterson's An Introduction to Decision Theory over Resnik's Choices and Luce & Raiffa's Games and Decisions.
Thanks for your post, it was a good summary of decision theory basics. Some corrections:
In the Allais paradox, choice (2A) should be "A 34% chance of 24,000$ and a 66% chance of nothing" (now 27,000$).
A typo in title 10.3.1., the title should probably be "Why should degrees of belief follow the laws of probability?".
In 11.1.10. Prisoner's dilemma, the Resnik quotation mentions a twenty-five year term, yet the decision matrix has "20 years in jail" as an outcome.
"I've never seen the Icarus story as a lesson about the limitations of humans. I see it as a lesson about the limitations of wax as an adhesive."
-- Randall Munroe, in http://what-if.xkcd.com/30/ (What-if xkcd, Interplanetary Cessna)
Harry could possibly decipher some of the meaning without asking. When seeing the original ("Thrayen beyn Peverlas soona ahnd thrih heera toal thissoom Dath bey yewoonen."), what did you make of it? I understood it was about Peverell sons and Death. The last word was somewhat reminiscent of German "gewonnen", but this Harry possibly doesn't recognize.