Semantically speaking, you can't teach yourself - only others. Teaching implies imparting knowledge or skill, or to give instruction to someone (other than yourself).
Fair statement, but...
You can gain insight on your own. Revelations or inspirations.
Einstein used imagination to produce his famous equation on relativity.
The apple (or some fruit) helped Newton and his idea of gravity.
It is not the same thing as teaching per se, but it is knowledge...
In most cases, all paths can lead to the top of the mountain. Some get there quicker. Some traverses hidden dangers. The main reason you get a guide to lead you up a mountain is to avoid those dangers and provide you with a quick and safe trip.
Body skills is slightly different to theoretical knowledge. A generalised version of this is to compare normal universities against polytechnics. Universities impart great knowledge, wisdom and articulate reading. Polytechnics gives its students practical experience and real grease time. Some pick up theoretical knowledge like a sponge and can ace an exam. The same person might know a car inside out as an engineer but might have problems replacing his spark plug.
A teacher or a manual can tell him how to do it, but in the end the person must learn on his own. So here comes the point... someone without a teacher or a manual could instinctively do this on his own too... by observation, elimination or experimentation. Mayhaps he would even be better at it because he didn't inherit his knowledge but had to learn by his own analytical skills, thereby imparting deep understanding. I know someone who is an ace programmer and taught himself how to do it.
Programming in a way is easy to learn on your own because by its nature, the laws are definite. As you understand the concept, its a matter of writing the language within those laws. Aikido is harder because it is covered by different laws. Laws of physics, anatomy, the natural universe laws and etc. Sometimes those laws are at odds with one another and it becomes a mess to understand.
I however stand by my earlier remarks. A teacher as a guide is necessary. But learning is your own responsibility.