@Willi: There is not "see" as "see"
Of course I have meant with mental and not physical eyes
@drsnuggles: This is a bit harder to explain...
When you imagine something, you see it with mental eyes, when you observe material world, you use physical eyes. If you want to observe spiritual world you need spiritual eyes (a bit simplified, but be it).
I have already told you that clairvoyance and OOBE sight is somewhat different, now let me explain in more detail.
Both clairvoyance and OOBE sight are different sight styles of spiritual eyes.
You can develop spiritual eyes by two ways (this is again a simplification)
1) Imagination with mental eyes (enough long and plastic imagination)
2) Using after images of physical eyes, good technique for that is Trataka (you look long enough into a candle flame and then you try to hold the afterimage in front of your eyes as much as possible).
Now regarding imagination:
When you imagine something, you "see" it, but not with your physical eyes (it doesn't even matter whether physical eyes are closed or opened). You perceive your imagination as if you really seen it, but that's not possible as physical eyes can see only physical reality.
Now about planes: There are four planes: physical, mental, emotional and spiritual.
I have already told you that you can develop spiritual eyes using physical or mental process. Here formula "As above so below" applies pretty well.
There is also another way of developing spiritual sight, and that lies on emotional level. You see, very strong emotion can also help you in development (Many people who lost their beloved ones got this gift to see them in their afterlives). Problem here is how to differentiate between these four planes. If what you see wasn't forced to your sight using your imagination and it's not based in either physical or spiritual plane, then only mental or emotional plane can be its source... That, in fact, means it is only a mere hallucination.
This post was very simplified but I hope it helped at least a bit