We have no way to define extinction—or existence—other than through the limits of our own perception. For many years, an animal was considered extinct a half century after the last confirmed sighting. The new standard, adopted in 1994, is that there should be “no reasonable doubt that the last individual has died,” leaving us to debate which doubts are reasonable.

https://www.newyorker.com/magazine/2018/07/02/the-obsessive-search-for-the-tasmanian-tiger