Originally posted by andrew
I voted that you can train by yourself, but I think there's a lot you can't do like this.
So, as the Bear in the Big Blue house says, "What do YOU think?"
Can you train by yourself and get anything out of it? Sure. Any time you become mindful and direct your attention in a focused fashion to some end you can derive something benficial from it. There are planty of solo exercises that are meant to suplement your regular practice.
Could you teach yourself Aikido? No. Apart from the issue of needing a partner with whom to do the techniques you would still fall far short because of the lack of a teacher.
You can work through books and videos and try to puzzle out what you see but you could spend years doing that and still have a highly developed system of bad habits and misundertsood techniques.
Each generation of teachers has spent many yaers developing a certain level of skill. They did that with the aid of their teachers who distilled their own experiences and past them on. Then the new generation takes that instruction, adds its own insights and passes it on to the up and coming.
If each generation were to have to completely reinvent the wheel each time there would be nothing at all possible like the sophisticated art we have inherited.