The phrase "existence proof" refers to a way of proving or showing that something exists. It is used to provide evidence or demonstration that a particular thing is real or true. Full definition
It's what mathematicians call a nonconstructive existence proof. (discovermagazine.com)
Steve, as far as the strip barks are concerned, I view your double - sampling of a single tree as existence proof that there are clear problems with such chronologies. (climateaudit.org)
At the most fundamental level, the signal gave them an existence proof: the fact that the objects came so close to each other before merging meant that they had to be black holes, because ordinary stars would need to be much bigger. (nature.com)