I think your use of the word 'exist' is a little fuzzy. What does it mean for a number to exist? Certainly, assuming that the universe is finite, there is some limit to the amount of numbers that can be implemented / represented physically. (Actually, we would probably also have to stipulate that all physical quantities, including time and space, are quantized. Otherwise, for example, for any number n, we could consider any region of space to be composed of n units of volume.)
But it seems that the notion of number is sufficiently abstract that to speak of a number's existence, one needn't refer to physically possible implementations of that number. To take your example, it may be that 10^10000 can't be physically expressed by a counting procedure, but it certainly can be
theoretically expressed-- after all, I have just theoretically expressed it using only 8 characters. In the sense that I can speak of it at all, it seems to enjoy some kind of existence, even if it's an abstract one.
gonzo said:
One can assume given the size of the universe relative to your brain, that there should be far more visual patterns out there than you have possible brain states.
This is an interesting idea. More relevant than the size of the universe, though, would be the number of retinal images that could impinge upon the retinas during the minimum salient duration of time needed to form a visual image. This gets a bit fuzzy, though.
On the one hand, you probably have significant information loss since the brain probably treats large classes of retinal images as identical. (For example, if you modify your current retinal image by adding a few photons of any visible frequencies, anywhere on the retina, you won't notice a difference. Thus for any given perceived visual images, we can already conclude that there are a large number of slight variations of the retinal image that are perceived identically.) On the other hand, you also have higher order gestalt visual processing which could interpret identical retinal images in a number of different ways. (For example, an ink blot literally looks different once you have decided that it resembles, say, a cow, even though the stimulus has not changed.)
Just for fun, let's estimate a rough upper bound for the number of possible visual conscious states possible in a given time period. One way to tackle this would be to roughly divide the visual field into pixels of minimum visual acuity and multiply the number of pixels by the number of noticeably different colors. But this method seems overly simple, as it neglects important higher order effects like perceptual grouping, foveal vs. peripheral vision, etc.
Another way to do it would be to count brain states, as you suggest. Assuming:
1) Conscious processes are supervenient upon brain processes;
2) A sufficient level of granularity to describe this supervenience is determined by the firing speed of individual neurons;
3) We can't a priori count out any part of the brain as having a potential impact on vision (although the visual cortex is most associated with vision, it seems to be influenced significantly by interaction with at least motor capabilities and higher order thought, and probably other many other capabilities supported in various brain regions);
4) The average human brain contains roughly 10^11 neurons (
http://faculty.washington.edu/chudler/facts.html);
5) The maximum firing rate of neurons is 250 Hz (http://64.233.161.104/search?q=cache:zTmPOk-t3hsJ:oxide.eng.uci.edu/publications/C1F05.pdf+%22maximum+neuron+firing+rate%22&hl=en )
6) Neuron firing rates only make a difference to conscious experience in discrete steps of 1 Hz. (There probably is some minimal difference beyond which neural firing rates make identical contributions to consciousness, given that at least some aspects of consciousness, like visual consciousness, apparently comes in discrete steps (eg http://www.nybooks.com/articles/17030), and it's probably lower than 1 Hz-- but our overestimate is going to be large enough as is, and assuming 1 Hz will simplify calculations.)
So if we have 10^11 neurons, each of which can assume 250 causally significant 'states' (firing rates) with respect to consciousness, then there are 250^(10^11) different ways to construct brain processes that are causally distinct with respect to consciousness in the span of one second. Of course, the vast majority of these permutations probably don't result in consciousness at all (ie when all neurons are cycling at 1 Hz), and in principle most of them are also not physically realistic since the cycle of any given neuron will be a function of the cycles of its dendritic neighbors, whereas here I assumed that they are completely independent. But there you have it: we can be reasonably certain that the number of ways to experience consciousness in the span of one second is less than 250^100,000,000,000.
