I rather strongly opine that the word 'exists' should not be applied to a state that can change with the vagaries of what formal system I happen to be thinking of.
The platonist certainly agrees. The test I described would only work for "accurate" maps of the territory. The platonist would consider PA to be an accurate (but incomplete) map of the actual natural numbers, while the formal system you described is not.
To answer your original question, the platonist need only point to a particular formal system (e.g., PA), and say that the nonexistence of the number 2 would mean that there would be no proof of 2's existence in that particular system.
This is at least an answer to the question, "What is meant by 'exist'?"; it gives us a definite procedure for deciding what does and doesn't exist. But I opine that it's not a very satisfactory one. Why that formal system and not some other one?
Actually, I was a little sloppy, there. When speaking on behalf of the platonist, I shouldn't have written "would mean", but rather "would imply". The point is that I wasn't defining what "the number 2 exists" means, but rather describing what the world would be like if the number 2 didn't exist.
At any rate, I don't mean to be giving a "definite procedure". For the platonist, PA is an accurate, but incomplete, map. Consider that there probably exist physical things of which we will never find any empirical trace. Similarly, the platonist expects that there exist mathematical objects that remain entirely unmapped by our formal systems or intuitions.
I still don't see the value in asserting on these grounds that something exists.
If you would be willing, on these grounds, to assert with some confidence that the computer will never print any more pairs, why would you demure from asserting, on these same grounds, and with this same confidence, "A largest pair of twin primes exists, and this most-recently printed pair is it."?
Why not stick with what is observable, namely that the computer halts, or doesn't halt?
(Just to clarify: The program I described never halts, even if the TPC is true. The program continues to run; it continues to brute-force factor progressively larger odd integers. It just never prints anything further to its output tape beyond a certain point.)
The platonist certainly agrees. The test I described would only work for "accurate" maps of the territory. The platonist would consider PA to be an accurate (but incomplete) map of the actual natural numbers, while the formal system you described is not.
Ok, but what is the test which distinguishes between accurate and inaccurate maps? It seems to me that the reasoning here has become circular: It is asserted that the number 2 exists because otherwise, formal system X would be unable to prove it; and also that formal system X is a good test bec...
I recognize the title could be more informative. At the same time I believe it says what is important.
I believe in a deity, I believe in mathematical entities in the same way.
The community of LessWrong (from whenceforth: LessWrong) is deeply interesting to me, appearing as a semi-organized atheist, reductionist community.
LessWrong seems very interested in promoting rationality, which I applaud. The effort does seem scattered, though, and this is the reason I post.
One has Eliezer's website with some interesting posts. The same of this community. The community links to some posts when you are coming for the first time into it, and you also have a filter for top posts. One has the blog. And recently, the center for modern rationality (in the same page as harrypoter fanfiction about rationality).
The point being there is no defined roadmap to go from AIC (average irrational chump to make an analogy to Game - which also seems to come up around quite a bit) to RA (again, rationality artist).
I write this post as to maybe generate a discussion on how the efforts could be concentrated and a new direction taken.
Should the creation of the Center for Modern Rationality envision this same concentration, this post may and should be disregard.
If it does not, then I leave it to your consideration.
Hang.