Agreed. In fact we can only prove this by inductive reasoning -
1. I have never experienced a sound in a vacuum.
2. Every test I apply and my theoretical understanding of physics suggests sound can not travel through a vacuum.
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3. Therefore sound does not travel through a vacuum.
However as Hume pointed out, I therefore have to assume the uniformity of nature, and that natural laws apply consistently at all times. I can not demonstrate said assumption. I have sneaked in another unevidenced premiss. If we assume the uniformity of nature (as I think we all would, but maybe not) - then this immediately gets pushed up to a deduction - otherwise deduction is restricted largely to dealing with formal logic and mathematical sets - but Hume doubted this, categorising our conclusion as a custom or belief,not the product of reasoning --
http://en.wikipedia.org/wiki/Problem_of_induction#David_Hume
Indeed, and if we are to follow Humes example that is precisely where we end up. No one has ever successfully managed to answer the Induction Problem, so to a sceptic like Hume almost all our beliefs are just that - many might fall in to the category Best Inferred Explanation, others are just inductive inference. Falsification has inherent issues as well.
None of this bothers me remotely as it happens - I just handwave it away, as I do radical Cartesian doubt - I assume there is an objective consensually experienced universe, other minds, and likewise i assume that there is a conformity of nature, in a pragmatic sense, and get on with it. Nonetheless technically the belief that sound never travels through a vacuuum is a belief that is not logically demonstrable and hence is arguably irrational, for the reasons stated. I doubt anyone will pick anyone up for claiming it, unless we actually develop evidence for the latter.
Of course it's logical to say incubi exist - if all your experiences suggest to you that is so. Years ago I met a lady who orgasmed whenever she lay down - source of many jokes when I mentioned it last time but actually distressing to her - and she blamed a ghostly lover. There was a simple medical explanation of course, and she was cured eventually, because I happened to have heard of a similar case through the SPR - but her belief sort of made sense in terms of her experience. There is loads of evidence for incubi - however here we have the simple question "is there a better explanation"? Parsimony suggest there is...
Anyway, here is my argument for theist being rational - note not theism - I'm a theist, and i seem to me be able to think and reason. I therefore conclude that theists can be rational, as can atheists, agnostics, and hell even Twoofers. Inded application of logic is what makes many people Truthers - but they lack sufficient data to make a clear judgment, or i do and they are right after all. Either way, as I keep saying
rationality is a property of an argument, not a group of people. I have on three occasions in my life bought a lottery scratchcard - and as it happens won - but I'm not going to claim that buying lottery cards is rational (actually it could be, in my situation*) - but that irrational action does not make me irrational as a person, anymore than all humans are irrational.
Why do I think
all humans are irrational? Because i believe in evolution by Natural Selection, and it follows logically...
1. The Human brain has evolved by Natural Selection for Adaptive Advantage
2. Adaptive Advantage may not favour understanding objective reality accurately
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3. Therefore humans may not be "designed" (for lack of a better term, no teleology intended) to understand truth.
I offer such well known concepts as Confirmation Bias, Outcome Bias, Expectation Bias - hell look at this list --http://en.wikipedia.org/wiki/List_of_cognitive_biases
It follows from our biology and how we got there that rationality is not something humans are going to be all that good at.
cj x
* Just to clarify the lottery example - if the odds of my lottery scratchcard giving me a break even win are 1 in 5, and a £200 prize 1 in 5000, clearly I am irrational to keep buying lottery cards. If however as I did I face a vet bill for £40, needing payment that day, and have only £1 to my name, the potential reward struck me as worth the gamble. I did and won, and paid the bill. £1 was useless to me - I had food etc. I needed cash - now. A large prize was worth far more to me at that moment than its cash value. Maybe to gamblers the thrill of the game offsets the economic loss, and to them the overall cost/benefit analysis appears rational, because the pleasure they derive from gambling losses is greater than the pleasure they would derive from spending said losses on other things. Dunno - but we must be careful restricting things to just economic analysis. A rural bus service may have a social benefit many times its economic profit value -- so we subsidize such services.
Professor Dawkins could argue exactly how evolution works (by his model) and make a perfectly logically coherent case: I could make a pretty good one which would suffice for non-biologists i think. That is a rational belief I guess. So the same true fact can be believed through rational and irrational methods... 
