Imagine a website devoted to talking about concepts. Let us suppose, in fact, that it's a phenomenally great website: it's full of interesting articles on interesting topics and you enjoy it very much (this supposition isn't actually relevant to the thought experiment, I just wanted to throw it in anyway).
A year into its existence, the founders of this website run away to the forest to live as monks. They have not yet paid to renew the domain name and have not given anyone the relevant passwords. Therefore, the site goes down.
Another year later, the founders return from the forest and the monkhood. After much sweat and toil and probably some more sweat and then additional toiling, these valiant individuals manage to create from scratch a website with exactly the same design and the exact same content. The philosopher Derek Parfit asks:
Have these people reconvened the very same [website]? Or have they merely started up another [website], which is exactly similar?
(Parfit used the example of a club, rather than a website, but the concept is the same; all future quotes are also replacing "club" with "website". From Reasons and Persons by Derek Parfit, Part 79 The Other Views).
Parfit says that this question might have a real answer: if, for example, the original site had made a rule stating whether it could be re-constituted if it ceased to exist. But suppose it didn't have any such rule, and suppose there's no other formal reason why we either should or shouldn't consider this new site to be the very same site as the old one. Perhaps you're thinking "well, does it really matter whether it's the very same site, or merely another site that is exactly similar?" Funny you should say that, because Parfit completely agrees with you. He says,
The claim "This is the same [website]" would be neither true nor false. Though there is no answer to our question, there may be nothing we do not know.
Parfit calls any question like this an empty question.
When we ask an empty question, there is only one fact or outcome that we are considering. Different answers to our question are merely different descriptions of this fact or outcome. This is why, without answering this empty question, we can know everything that there is to know. In my example we can ask, 'Is this the very same [website], or is it merely another [website] that is exactly similar?' But these are not here two different possibilities, one of which must be true.
If you're now thinking "well, this is quite pedantic philosophising" -- would you say the same thing if it applied to you? Most people think personal identity must be determinate: either you exist or you don't exist, either you are alive or you are not. But Parfit disagrees: he thinks there can be cases where "Am I about to die?", or "Am I the very same person as the one in that photograph, or merely another person who is exactly similar?", are empty questions. And this has pretty fundamental implications for how we think about our lives, and our eventual deaths. But that's another concept…