A resource is identified by URI and may emit representations. There’s no way to tell from the representations what the resource “is”; I tend to believe a resource is what its publisher says it is as a good rule of thumb. But it doesn’t affect the software very much.
–Tim Bray on the xml-dev mailing list, Wednesday, 23 Jul 2003
REST is like Quantum Mechanics; or, more specifically, resources are like atoms (and I do mean atoms, not ATOMs). In quantum mechanics you cannot actually say what an atom is, where it is, or how fast it is moving. You can only predict how it will respond to certain experiments, and then only in a probablistic fashion. As soon as you try to figure out what the wave function actually represents, well then you fall down a sink hole of bad physics and worse philosophy.
Resources are the same. When designing RESTful systems, you never see the resources. All you see is the URL and the representation of the resource the URL provides. What resource does the URL identify? Who knows? The only way to reason about it is through the Copenhagen interpration.