In partial answer to the questions posed in the last post, I found an interesting paper that shows how, given any logic (within certain broad boundaries), it is possible to construct a corresponding probability theory. In particular, this paper uses the technique to construct an intuitionistic probability theory.

The paper also argues against abandoning what it calls the "principle of addition". The principle is simple: given non-overlapping states, A and B, the probability of A or B should be the probability of A plus the probability of B. (Formally: if P(A and B) = 0, then P(A or B) = P(A) + P(B). The paper gives a slightly different version.) This may not sound very worrisome, but it actually causes severe problems for the frequency interpretation of probabilities. (I was very surprised to learn this. The idea that probabilities are frequencies is logically incompatible with the standard rules of probability theory! I have not yet found a detailed explanation of the incompatibility; it is often mentioned, but rarely explained...) So although a large class of alternative probability theories are covered by the definition (as many as there are alternative logics), the definition is very conservative in some ways.

I do not know much about what happens when we drop the principle of addition (also commonly called "finite additivity"). But in some sense, fuzzy logics are among this field of possible notions of "probability". This idea (together with the fact that frequencies are actually incompatible with normal probability) makes my slightly less antagonistic towards the fuzzy logic approach... but not much. I was previously of the mindset "Probabilities are the mathematically correct way to go, and anything else is just silly." Now I am of the mindset "Probabilities have a fair mathematical motivation, but there is room for improvement." So another theory may be workable, but I'm asking a very strong foundation before I'll stop clinging to standard probability (or if I finally settle on an alternative logic, some conservative generalization of probability such as the one above).

In other news, I've found some very interesting material concerning intuitionistic logic. Sergei N. Artemov has done some amazing-looking work. In particular, on this page, he explicitly claims to have a logic that "circumvents the Incompleteness Theorem". I don't yet know exactly what he means, but the approach is described in this paper, which I will read ASAP. (Actually, I'm reading this one first, because it is also very interesting...)

I have also been running into some amazing math blogs lately. Good Math has a nice introduction to lambda calculus and intuitionistic logic (as well as pleny of other introductions to important areas). (Note: the blog moved from Blogspot to ScienceBlogs, but the old posts I'm referring to are the ones on Blogspot.) Also, Mathematics and Computation looks like a good read. There is a very interesting-looking post on seemingly impossible functional programs, which gives an algorithm for exhaustively searching through the space of infinite sequences of 1s and 0s. Unfortunately, I did not understand very well on my first read, because the algorithms are written in Haskell, which I am unfamiliar with... I found this blog through Machine Learning (Theory), another good blog.