Without lightness, everything the horse does is unnatural. Lightness is a necessary requirement for reach that ‘state of grace’ which is indispensable in art.
Nuno Oliveira, in Horse and Rider: Annotated Sketches (with Jean-Louis Sauvat)

All sorts of excuses are offered to explain a failure to put the horse ‘on the bit’. The horse is said to be too young; he has only been out of his stable for ten minutes and has not yet agreed to come on the bit – and many others. Needless to say, all such explanations are valueless, the simple and unequivocal fact being that once the horse has begun his work, no excuses are valid and only poor horsemanship can explain the failure. There is never any necessity or excuse for a human being to move around in poor posture once he has left his bedroom in the morning. The same principle applies to the horse.
Anthony Crossley, Advanced Dressage