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The war and the Spring, that broke out almost simultaneously, both distract me from philosophy. I also have to think about where to go when I move out of my flat in about two weeks time. Should I stay in Berlin and enjoy another cheap and relaxed summer, or should I rather go to Bielefeld and enjoy some reasonable philosophy? Unfortunately, in Germany the quality of philosophy departments is inversely proportional to the attractiveness of the cities where they are located.
Frege uses second-order quantification in both his formal and informal works. So far, I have always assumed that his second-order logic is standard second-order logic. But couldn't it also be second-order logic with Henkin semantics, which would in fact be a kind of first-order logic (compact, complete and skolem-löwenheimish)? Unfortunately, I know far too little about second-order logic to answer this question.
Are there any second-order statements that are satisfiable in standard semantics, but not in Henkin semantics? (I guess there must be: Wouldn't second-order logic with standard semantics have to be complete otherwise? Not sure.) If so, do any of Frege's theorems belong to these?
I've finished the exercises. I still have to put together some of the solutions, but since Word always crashes when I draw complicated tables and trees, I've decided to take a break in order to save my mental health. (In fact, Word not only crashes frequently in these cirumstances, it also deletes the currently open file while crashing.) So now I'm working on the Frege paper again, which I really want to finish soon.
Brian Weatherson has posted a couple of interesting entries on imaginative resistance.
I've finally managed to introduce the provability predicate and its properties without mentioning representability and recursiveness. The exercise is then to derive Löb's theorem and Gödel's incompleteness theorems. Unfortunately these deductions are not as simple as I thought they were. Probably too difficult for an introductory book.
I've also just made up this puzzle, which is not very difficult I think. ("Not very difficult" even in the ordinary sense of "not very difficult", not only in the David Chalmers sense.)
I'm still doing exercises for the logic book. This is rather unpleasant because I have to use Microsoft Word. Getting back to Word after using reasonable document formats (like LaTeX) and editors (like Alpha) for a while is a very frustrating experience.
At the moment, I'm trying to find nice and simple versions of Gödel's Theorems that still leave something formal to prove (like deducing Löb's Theorem from provability properties). This turns out to be difficult because I don't have the space to introduce the concepts of representability and recursiveness.
First, the puzzle:
In a certain country there are two Gods, called A and B. One of them (A or B, you don't know which) only tells the truth, the other one only falsehoods. One day you meet a God in this country and want to find out whether it's A or B. You're only allowed to ask a single yes/no question. Unfortunately, you don't understand the language of the Gods (even though they understand yours). All you know is that their words for "yes" and "no" are "qwer" and "poiu", but you don't know which of these means "yes", and which "no". With what question will you be able to find out whether it's A or B you're talking to?
I can't really say that I have made up this puzzle. Well, I have made it up, but I took all the main ingredients from puzzles by George Boolos, who himself owes them mainly to Raymond Smullyan and a computer scientist whose name I forgot.
My logfiles show that an alarming number of people (namely more than 10 per day) look at my blogger and diffbot scripts. Don't do that. Don't use them. I have almost finished programming much better versions of both that might be worth trying. But since they have been in the "almost finished" state for quite some time now, I thought I'd better add this note here.
Apologies in advance for another somewhat political entry.
Given the current state of everything, there is little hope that the easily available oil reserves won't be exploited until they are eshausted. There is also little hope that anything like the Kyoto protocol will prevent this from happening rather soon. For well-known reasons this is not good at all. So maybe those governments and institutions that care about the environment should try a different strategy: We could move forward with the exploitation of oil reserves, but instead of burning the oil and blowing all the carbon dioxide into the atmosphere, it could be bound in plastic.
On a more positive note, Prof. Beckermann found my speculations convincing: The next edition of his logic book won't contain "incorrect" quantifier rules any more.
I can't count how often I wished to live in that very close possible world in which Al Gore won the presidency (very close in terms of Lewis' similarity standards for counterfactuals, not in terms of overall similarity, sadly).
What worries me most is how many US Americans seem to back the Bush administration. I mean, when Clinton, like so many other people, had an extramarital affair and lied about it, that was a big scandal and caused an impeachment process. When Bush, quite unlike most people, violates the UN charta by going to war against a country that doesn't threaten the US at all, and keeps lying about his alleged knowledge of Iraq's weapons of mass destruction and links with Al Quaeda, Americans just seem to buy it. Then again, they are renaming French fries. There goes my princple of charity... For now, I blame it all on the absence of a free press in the US, but I'm not sure if that's a sufficient reason. I also tried to read blogs of the war-mongers, but that didn't help much, it just left me very depressed for the rest of the day. It's like shopping at Kottbusser Tor.
Oh well, I should better be blogging about quantifier rules in axiomatic systems of predicate logic.
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