Among the most frequent phrases you read/hear in trading forums is 'there is no such thing as a holy grail. The item is no one has (yet?) Tried to accurately specify what a holy grail might be before saying it does not exist.
It is a bit like a theist saying God exists! As well as an atheist saying, god(s) does not exist! When neither one can agree about what exactly they mean by the term. Can it be a man with a snowy beard in the skies? A superior alien? The Divine Creator of Everything? A key cause? Something unknowable?
So it is with the Holy Grail of trading - can it be a system that never ever loses? A system that generates the maximum yield potential with the least risk? Together with the tiniest investment? What would a system have to be in a position to perform - for you to say 'yes, in the last, the grail has been found'. And afterward, is this the end of inefficient markets because we know them? What would disqualify a method out of becoming the holy grail and only leave it a 'system worthy of trading, but could be improved'? A 'grail contender' if you may.
Of course a definition won't stop people from saying that they possess the holy grail or that it can't be accessed, but it may be interesting, possibly even helpful, to know when a system is so perfect that to tamper with it any further would be to kill the golden goose.
History
Probably everyone knows the story of the holy grail comes in English and French Arthurian legends. The grail was that the cup that Christ used at the Last Supper and in the event that you could find this holy relic, it would grant you immortality, or anything comparable. You probably Keep in Mind That scene in Indiana Jones and the Last Crusade. He chose poorly.
The knights of the round table searched fruitlessly for the grail, and also the person who came closest, (Percival? Galahad?) Merely got a glimpse of it before it vanished. The other knights died or came back to Camelot empty-handed. So, inherent in the idea of a grail is the idea that it can never be accessed. It should always be just out of reach. That questing for it is fruitless, and potentially perilous. I presume it is the exact same in trading. A holy grail system is one that is so good, it can never quite exist. But a 'practical grail' would be one that is as close to the ideal as you can.
Profitability and Performance
In your view what would a grail system have to be in mind?
Minimum win rate?
Minimum Sharpe ratio or profit factor?
Maximum drawdown?
Maximum 'value at risk' ?
Minimum expectancy?
Minimum return on investment?
Minimum Compounded Annual Growth Rate? (CAGR)
Minimum or maximum account size?
Minimum wait period between profits?
Minimum number of trading opportunities?
Does this need every one these, a few of them, or something else entirely?
Likelihood
We know for sure that there are trading systems. We know that some of these approaches are inherently better than others any metric you could choose. There are a few subjective qualities to what makes a good trading system, but if we presume the aim of trading is to make profits, a few systems will only be more profitable than others. When we knew all the systems in existence, and understood their expectancies, then we can rank them, and choose the best among the best. Can that top system become the 'holy grail'? Can it be close enough to be a 'functional grail'? In my opinion the answer to the last have to be yes. Thus the grail probably does exist, or at the very least, the most performant system that can reasonably be invented by man has been made and has been used to exchange. This is the method against which any future grail contenders have to be compared. What does it resemble?