"The most important thing is to not stop questioning."
An impetus to the development of our training system is an increasing disillusionment over many years, with some modern training methods, such as "Push and Resist," "Drive and Hold," and the use of coercion, maintenance beating, and gadgets.
That is not just an admonition to be imaginative or creative or whimsical, but also to draw on other areas of expertise—the sciences (particularly physics/mechanics, veterinary science, behavioral science, and physical therapy) and other disciplines (martial arts, dance, Pilates, vaulting, skating, and dog, dolphin, or parrot training).
Layman-friendly publications, such as Nature, Science, and Scientific American can sometimes yield more insight than a dressage treatise titled How to Do a Perfect Half Halt. Much insight can be gleaned from sources written by Dr. Hilary Clayton, Dr. Temple Grandin, Dr. Gerd Heuschmann, and other experts in related fields.