(I'll continue playing the devil's advocate .)
That is what the "artist" want you to believe: that they were always artists, that they always had this "vision" which raises them above the crowd of mere mortals and that what now hangs in a museum always had this peculiar quality which started before they were recognized.
The truth is that one can't prove any of this, because the definition of artist is self-referential. Is an artist whoever manages to have their works hang in a museum, the definition is simply not valid before that point. Saying they were artists before that point, they always had this peculiar quality in themselves, simply violates the definition.
And, in truth, known, internationally recognized artists often have a period of experimentation where they try many different things till they find a style which sells. They cannot have had a "vision" when history proves they were seeking one, can they?
This system of museums that collect art, do so for posterity and also, where funded by benefactors, to attract more financial support. So the curators, (in addition to finding the exceptional art worthy of saving), might also dealing with aspects of fashion and so
all their selections may not be justified long term. Also they likely as not miss some artists they should have collected.
So museum acquisition is merely an excellent practical definition of the finest of art, but not a logical definition, like "up" is opposite to "down". Also the measure of vision in the collected or rejected art is not made by acquisition. Rather, as a group, museums will tend to collect much of offered or available works that often have unusually strong vision. Included will be some recognized work that clearly represent samples of the best craftsmanship, but little vision and the reverse. The best example of the latter, in Duchamp's iconic signed and repurposed "Urinals" to which he contributed no craft of forming the art. He just signed and repurposed an off the shelf object. It happened to be a curvaceous white porcelain object of considerable evolved generations of simplicity. It was not made from his imagination but in a mass-production by unknown factory artisans.
What
is collected does have a better chance of long term survival for society and is a rough representation of the best of our Art. Work that is not collected is not
necessarily "not Art" just not art that's recognized right now. Similarly, what is collected is not necessarily possessing the greatest vision nor does it mean that what's excluded lacked that quality. Rather, as a whole, many of the collected pieces do seem to be extraordinarily enriched in vision.
Still, I assert that all people who imagine and export their thoughts to material form in such a way as to realize the experience of the feelings and reactions they hoped for from the work are indeed artists. It may not go further than that. It just depends on the work getting known enough and infectious to reach collectors and move them sufficiently. If that person has patrons, connections or is a celebrity already, then this climb to value might be a lot easier. In our open societies, exceptional vision is, (we would hope), recognized often enough that many of the best works do indeed arrive at a museum to be preserved for the rest of us.
The final "practical definition" of "Art" going forward is what is collected by museums and collectors, even though we know that this is not an exclusive definition, as there's much extraordinary work that never reaches collectors or museums, again for a myriad of reasons.
Parallel to this are many works that sell in countless art galleries as "Art". Some do seem to be as worthy as what's collected. This layer of work is still Art, just not as yet all recognized as the finest art by museums that, by doings so, give a stamp of reassurance to private individuals, that the work is likely worth the investment. Below this is the product of self-proclaimed "artists". Yes, they can still be called artists, even though their work never gets to a local or national gallery. Imagination exported in an evocative form can be and is often art. Just may not have the magnetism to attract anyone but the artist and his cat.
Asher