Shoot, I asked this several years ago over on AikidoJournal's forum and got a bunch of stock responses.
Frankly, I don't know anymore. No one has been able to offer me satisfactory answers to "What is Aikido." All of the things that are brought up as unique, I've found in other unrelated systems. I generally think of it as an off-shoot of Daito Ryu now. Perhaps OSensei really did transcend all that, but since none of his senior students seem to have understood what he was talking about, how are we ever going to find out? The closest group would probably be Inoue's Shin-ei Taido, but they don't consider themselves aikido either...
I thought that Morihei Ueshiba was really pretty clear in "Take Musu Aiki", that's what he spends most of the book discussing...
If you don't know what it is, or what the goals are, then how do you know that practice of it requires physical training methods?