What is the single most important principle in all of dressage?

If forced to identify a single most important principle that underlies all of dressage — the one from which all others follow and without which none of the others can be correctly understood or applied — it is this: the horse must always be able to go genuinely forward. This principle, captured in Steinbrecht's famous dictum to ride the horse forward and make it straight, and echoed in every classical tradition from Xenophon through de la Guérinière to Oliveira, is the foundation from which every other quality in dressage either develops or fails to develop depending on whether this foundational principle is honored. Forward, in this sense, does not mean fast — it means the horse's genuine desire and ability to move energetically in response to the rider's aids, to seek the contact with forward energy, and to maintain its gait and impulsion without constant driving. Without genuine forward, rhythm cannot be regular because a horse that is behind the leg does not maintain a steady rhythm; suppleness cannot develop because a tense, reluctant horse cannot swing through its back; contact cannot be genuine because a horse without forward energy cannot seek the bit with the energy that real contact requires; impulsion is impossible because impulsion is precisely the elastic, forward energy of engaged hindquarters; straightness cannot be developed because a horse that is not genuinely forward will compensate through crookedness; and collection cannot be achieved because collection requires the forward energy that is redirected and stored in the carrying engagement of the hindquarters. Every problem in dressage ultimately traces to either a loss of forward or to forward without the gymnastic development that channels it correctly, and the restoration of genuine, willing forward movement is the beginning of the solution to virtually every training problem that dressage training encounters.