Psy-Kosh: Maybe I really tried to approach the meaning of the question from the direction of subjective experience. But I think that the concept of "existence" includes that there is some observer who can decide if that thing we're talking about does really exist or doesn't, given his/her stable existence.

Maybe that's why the question can't be easily answered (and maybe has no answer at all) because the concept of "world" includes us as well. So if we want to predict something about the existence of the world (that is what the word "why" means, I think), we haven't observe anything: it's a logical truth that any world in which this question is asked really does exist.

But if we statisfy the two assumptions in the question (the existence of the world is observable by us, and is repeatable, so we can make predictions about it), it starts to make sense, but it becomes less mysterious somehow. Some possible answers: because the previous one did already collapse in an anti-big-bang, I've just seen that..., or we usually have to wait for it to recreate itself, therefore nothing exists right now... Or it exists because I was bored and created a new one, or maybe because I was bored and started Half-Life (which also fits our new world-concept in some way)... etc.

And... physical equations are definitely _something_ which differ from nothing. Some rules for a... world... But I think if something is becoming so blurry like the concept of "world" just now, we better ask what subsystem in our mind is applied for the wrong problem, and what are those problems which it is intended to solve.

Psy-Kosh: let's Taboo "exist" then... What does it exactly mean? For me, it's something like "I have some experiences, whose cause is best modeled by imagining some discrete object in the outer world". The existence or non-existence of something affects what I will feel next.

Some further expansions: "why": how can I predict one experience from another? "world": all the experiences we have? (Modeled as a discrete object... But I can't really imagine what can be modeled by the fact that there is no world.)

So the question "why does our world exist" becomes something like "what is the experience from which we can predict we will have any experiences at all... Sounds a little bit more controversial than the original.

By the way, have we tried this transformation in the opposite direction? Turning the question "what is the sound of blue" to one which seems to make sense...

@Roko: The visual cortex isn't the only one thing we use. Other parts of the brain probably "cache" some of the insights gained by visualizing things, or trying / imagining movements etc., also common sentences, so we can use these areas for other things we've never seen before. These cached things are our concepts, I think.

You're right, I won't visualize every part of the thought "technology advances exponentially because technology feeds back positively on itself". But I've seen a lot of exponential functions in math classes, plotted them on screen, and noticed that they can grow very big. Now I use this concept for understanding this sentence. It would be hard to explain this to a five year old, or to somebody who has never seen exponential functions: you can't visualize so many things at once, without using any cache mechanisms. (That's why inferential distances are so long in reality, I think.)

With only language and the cached thoughts (grammar / logic and rules in a symbolic system) we can get surprisingly far, but not far enough. (For us, even logic is a cached thought from the visual cortex, for it describes the connections of distinct things. This is a special feature of vision: try to imagine two songs at the same time...)

Are words really just pointers? If you want to refer to objects which you've visualized, they indeed are. But people even do some peculiar "arithmetic" with words, forming sentences, which has nothing to do with meanings.

For example, when I'm sleepy (half sleeping state), sometimes I notice that whole sentence structures are running through my head, without the words filled in, but I know where the sentences begin and end, and how they are connected. Even specific words show up time to time, but the whole stream has no sense at all. But if you don't visualize and use concepts... it just sounds right.

So I think words as stand-alone things (with their sound and syntactical role) also have an important role for connecting those things in a more abstract way, whose connections can't be inferred only by visualization. (Think of a linked list of pointers: the position of the pointers in the list can be as important as the referenced object itself.)

Roko: FOPL is similar to the taxi driver who never visualizes anything. (It never dereferences the pointers.) I don't think the solution would be a much better symbolic system (although FOPL is not really designed for dereferencig), but to connect a visual cortex to the symbol manipulation system. So the similarity of two symbols could be checked by simply visualizing them.

Unknown: What do we mean by "chance"? That it has a very small a priori probability... The evidence is given: the two sequences are similar. We can also assume that the evolution theory has a bigger probability a priori, than the chance to get that sequence. These insights were all included in the post, I think. So applying Bayes' theorem we get the fact that the evolution version has much bigger a posteriori probability, so we don't have to show that separately.

There are a lot of events which have a priori probabilities in that order of magnitude... But we also should have strong evidences to shift that to a plausible level. But a lot of people think: "there was only a very little chance for this to happen, but it happened => things with very little chances do happen sometimes."