That's what I meant by tautologically useless. You might be able to test it for commodities like bread or crude oil, but you cannot for anything remotely differentiable. A recent example:
Apple supplies tablets. There is great demand. Market participants conclude: "If we supply tablets, we can profit of some of that demand". Market is filled with tablets at various price ranges ($50-$700) with various capabilities. Almost none of them sells.
Conclusion: there is demand for ipad, no demand for tablets.
Any theory that requires you to retroactively define your terms is "not even theory" in my book.
The Kindle Fire in particular is selling well, primarily because the Amazon media experience is almost as good as Apple's. I don't think that's tautologically useless: there is a market there, but apparently you need to spend a _lot_ of time writing the software - Apple had a decade to get iTunes to where it is today.
Amazon seems to understand that and has taken total responsibility for the software on the Kindle Fire, to the point they forked Android. I think that's a mistake long-term but short-term they get this one thing: Apple's hardware comes with Apple's software, and it is the combination that is so valuable.
P.S. It took me forever to research these tablets and post this list. The experience of trying to find a decent Android tablet is incredibly frustrating (unless you just go straight to the Nexus 7). The prices are all over the place, which to me indicates junk marked up to catch unwary buyers. The reviews are pretty scathing about poor build quality, unusable Android builds, and horrible customer service. All of that has an effect on the marketplace.
Apple supplies tablets. There is great demand. Market participants conclude: "If we supply tablets, we can profit of some of that demand". Market is filled with tablets at various price ranges ($50-$700) with various capabilities. Almost none of them sells.
Conclusion: there is demand for ipad, no demand for tablets.
Any theory that requires you to retroactively define your terms is "not even theory" in my book.