To be fair to Parilli, he is trying to teach through the media of books and DVDs, largely inexperienced owner/riders how to school their horses. He explains to them that all horses have a personality and that one standardised training system, stuck to rigidly by the trainer, won't necessarily work on all horses. Previously in the UK the British Horse Society had produced several manuals on how to train a horse but they were very basic in content and did not take into account the differences in the type of horse - or just as importantly, the personality of the human trainer.
Michael Peace in his book 'The 100% Horse' divides horses up into 11 types
from "Anxious" thru to "The Trader" MP suggests that before the owner makes any attempt to train he should first ascertain what type of personality the horse has. He doesn't go so far as to say which side of the horse's brain is switched on but nevertheless he is in principle following Parilli's concepts
If it is generally accepted that horses vary in personality, as indeed they do, and that people differ in personality, as they do, then quickly it becomes obvious that some horses will never suit their owners because the differences in their personalities will clash. Brash won't ever mix with timid.
In the olden days, the professional horseman would use a standardised method of training, one designed to enable the horse to be used in a short space of time. The professional's ability to sit even a difficult horse meant that he could always get out of a horse most of what he wanted. Nowadays the amateur owner/rider seeks more - he/she seeks an harmonius relationship - which the professional never sought although no doubt he would have valued such a bond as and when it was acheived thru time.
Before WW1, the gentleman was presented with a well schooled horse to ride, fit for purpose, which had been brought on by the Head Groom of the estate.
The rider was not concerned with schooling (or mucking out).
So when we buy a horse, we should perhaps try to better assess the horse's temperament and personality and we must ask ourselves whether the horse is fit for our purpose. The cost of the horse is irrelevant in the selection process but of course it does figure in the decision to buy.
It occurs to me, that immediately we start to assign emotions to horses, the horsey life gets complicated - which no doubt was the reason why in the olden days one did not seriously attempt to see horses as anything more than a beast of burden.