Sure, ever heard of Sokaku Takeda?
Anyway, there have been quite a few definitions of IP/IS over the years, if you search through the forums. There's also a pretty good summary at http://www.internalartsinternational...-internal-art/
For "It has to be felt" - well, it's pretty had to teach anything in a detailed manner by correspondance - try doing it with conventional Aikido and you run into the same problems. The more subtle and complex the skill the harder it is.
Well, there is a contradiction for you. The link that you gave here leads to an excellent introduction of the inner arts. Several of the important concepts are mentioned and even a bit explained.
So it proves my point - it can be discussed.
It can also be described, just as we can describe just about anything in this world.
It proves another point - several proponents of IP / IS do not seem to know these basic concepts (as mentioned on the website) or are even against it. Some have even expressed this clearly in this very thread. At the same time they claim knowledge that nobody else has. That is a contradiction too.
If someone does not understand these basic concepts then a more modest attitude would be more fitting.