The phrase "a demonstrable fact" is correct and usable in written English.
It can be used when referring to a fact that can be proven or shown to be true through evidence or observation.
Example: "The climate is changing, and this is a demonstrable fact supported by extensive scientific research."
Alternatives: "an evident truth" or "a verifiable fact".
It is probably a scientifically demonstrable fact that anger and disgust are more effective for the motivation of people writing letters to Sunday newspapers than, say, the desire to congratulate a writer on a well-penned piece of prose, or even approval of the sentiment expressed in a column.
Arendash's interesting experiment still needs to be replicated by other researchers before it can be seen as a solid, demonstrable fact.
For something that is constantly and passionately called into question, the prevalence of rape in our society is an easily demonstrable fact.
It passes muster as a metaphor if not as a matter of demonstrable fact.
Well, yes, the analysts say, obviously you have to adjust the numbers for the circumstances — if your minor-leaguer is hitting in a league where everyone hits.300 it means less than if he's doing it in a league where everyone hits.150 — but the numbers are, as a matter of demonstrable fact, consistent.
If he has studied the history of textual criticism he will know that as a matter of demonstrable fact nearly all conjectures are wrong, and he will accept that many of his solutions are in the nature of things provisional.
First, its primary conclusions are entirely false as a matter of demonstrable fact.
Yet, somehow claims of progress, not backed by any demonstrable facts or tempered with any degree of sincerity, such as the assurances Congressman Pence provides, continue to be proclaimed with enthusiasm and a veneer of honesty by leaders throughout this country.
