Here are some lovely quotes from Workbooks From The Spanish Riding School (1948-1951) by Charles Harris:
"All the old arts are new when discovered by each individual for the first time."
"Energy is both mental and physical exhilaration."
"Relaxation cannot be confused with fatigue."
"The higher the head carriage in the canter, the more the horse can spring under the rider in a clear, correct gait and tempo."
"Horses can resist correct extension on staight lines by stiffening the spine lengthwise, but not on a circle."
"Almost anyone can do the exercises in a complicated bit, but it takes an artist to do the simplest exercies, correctly and accurately, in a simple bit."
"For dressage to be an ART there must be an equal balance and consistency on both reins."