> "imagine a number" and you not being able to tell me any digits of the number - you didn't really do what I asked.
I don't really feel like that's true. If you asked me to choose a specific number, sure. But if the task really were just to imagine the concept of a number, unbound to anything, it wouldn't be a specific number until it needed to be.
Similarly, when thinking of a cup, unless I'm explicitly trying to imagine a specific cup, as close to reality as possible, it is merely a cup. Not a red cup, or one with a tiny annoying handle that I can only fit one finger into.
There are two questions there. Select the properties of a randomly chosen cup, and relay them to me. At which point i'd feel like im making it up as you ask for specific properties. Or just picture a cup, squashing, stretching, rotating. I think very much that these mental images are deeply baked into the ways we interact with the world (and are not literally seeing so much as attention to details of idealized concepts). It can't be a coincidence my mental image having transformations applied to it effortlessly is because of my work with 3d modelling and robotics.
If my mind wanders into those things, I could imagine the cup morphing between those scenarios, unreality forming into reality. Seeing with the minds eye is not seeing, in a literal sense.
I believe that there is a sense in which if we were forced to actually see the real world as it is, every detail of everything all at once, as a camera sees it, it would be far too much (consider things like optical illusions, human blind-spots in the visual field). Hence all sorts of mental shortcuts are taken where below the concious layer things are stiched together to make a false sense of cohesiveness out of a distinctly disconnected and incoherent stream of input data. Every sense even has varying time-lag, to the point the brain predicts nerve signals before they come in, hence phantom limb pain etc. It ends up that so little of you is actually the concious mind you.
Objects in the minds eye are similarly abstract objects. You cannot see every detail unless you decide upon what you choose to see. An apple, in my mind, has the blotchy, slightly coarse texture on the outside most apples have. But is it a red apple or a green apple? It's somewhere between the two. The digits of my number are a blur approximating all digits, the digits I expect to see, until I look closely. Similar to how the room in your peripheral vision is still the same. Probably.