I don't post here very often, so I hope no one will mind if I jump in here, as this is a very interesting topic and one that involves a "stumbling block" if you will to my complete interpretation of Satanism.
My own interpretation of Greater magic is the same as that of Lustfire and The_Lightning; that the ritual serves as pure psychodrama. I have no problem with this at all. The idea that real world events can be altered by our activities in the ritual chamber is one that I do have difficulty accepting. I will confess that I have never carried out a Satanic ritual and thus the rest of this post is purely conjecture, and I completely accept and agree with anyone who believes my views are non-valid for that very reason. However if someone could come back to me on these views I would be very grateful.
I am a scientist by training and also in my career, and I believe that the scientific method is the ultimate guide to testing our world around us. There are testable absolutes, and science is no place for relativism. The speed of light to a stationary observer in California is the same to a stationary observer in Finland; no matter the intelligence, education or even physical and mental state of the observer.
What happens when you are outside of ritual walking along, and all of a sudden you see the result of a ritual’s success beyond what could be attributed to mere self transformative psychodrama. Something like, a person rattles off word for word the dream you scripted for them to have. The usefulness of self transformative psychodrama is wonderful in and of itself, but when a result pops up outside of the temporary “suspension of disbelief” your rational mind can not help but accept that something is happening there. Having this sort of thing happen often enough will create a concrete assurance that the “supernormal” is happening, but not necessarily the “supernatural”. Sometimes our preconceived assumptions of our physical limits just need adjustment. You can test where the limits are by trying it out yourself.
From my point of view, these sorts of events would not be enough to prove that a ritual worked. We humans are pattern recognition machines, and the sheer psychological power of a satanic ritual would be enough to send me unconsciously seeking for little signs of its working in my everyday life, whether or not I actually believed it could occur. For example, I do not believe in ghosts; in fact I'm certain they do not exist. However ask me to spend a night in a creaky old "haunted" house alone and I wouldn't be able to stop myself from being slightly unnerved.
It is for this reason that I believe we have to tread carefully when using personal experience to prove something. There needs to be a definite objectivity of measurements to prove that no personal bias affects the results.
To prove that a ritual had worked, I would need double blind tests of the ritual, from various individuals. I am aware how absurd and impractical this would be and I'd have no idea how to go about it, but I'm using this as an example of the rigors demanded by scientific experiment.
Also, reading the Balance Factor section of the Book of Belial, I became aware that the impossible is not possible with greater magic. It seems that (and perhaps my understanding of this section was not fully clear, so forgive me if I am incorrect) you can only achieve in a ritual that which you are capable of in real life. For example, I would assume that no matter how hard I tried in a ritual, I would find it impossible to make myself fly, as this violates both human anatomy and physics. It would be silly of me to expect such from a ritual. I would have more success carrying out a ritual for more achievable ends. And, if we can achieve these things without the help of greater magic; surely it’s better to admit that we did it without it? That we are in fact successful, enterprising, talented individuals who can achieve many, many things when we put our minds to it?
If I have gone too far and am poking about in things best left for the members section; please forgive me.
Edit: Pardon my silliness, LeviathanXIII's post clears up quite a bit of my issues. I'll be sure to not hit "post" next time without checking no one else has answred my question while I was typing.