A great deal of what we learn is self-taught. We learn through modeling, observation, trial and error, and pattern recognition, and we do all these things with or without the help of others.
As evolving humans, this is what we do. As many philosophers of education point out, we are hungry for learning and will naturally develop many important skills and realms of knowledge with no more than the slightest nudge from those around us. In a very true sense, we are students of our world, exploring, problem solving, and acquiring knowledge and understanding every minute of every day.
Self-teaching may not be considered a practice or a philosophy, but any discussion of teaching and learning that does not reflect its importance does little justice to the wonderful capacity for development and evolution built into the human brain and body.