system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: I read online something about how 85% of Earth's species are still yet to be discovered. How do we know that we've only discovered 15% of all species when we don't know what "all" is?
gpt
Take a bunch of samples from all over the world, look at them really carefully and count how many different species you find in them. 

Some of them you will already know, and some will be completely new. 

If in 1000 samples, you find 100 different species, of which you only knew 15 before taking the sample, then you now have your answer as to why we estimate only 15% of species have been discovered.