Right. Another way to think about bloom filters is a data structure that remembers a list of hashes. It will tell you "oh I saw this hash before". What it will

Right. Another way to think about bloom filters is a data structure that remembers a list of hashes. It will tell you "oh I saw this hash before". What it will NOT tell you is "oh I saw this element before". If "Foo" and "Bar" hash to "XX" and you've shown "Foo" to the filter, it will think it saw "Bar" because what you are really asking is if it saw something that hashes to "XX". That's the false positive. On the flip side if "Baz" hashes to "XY" then the filter would tell you with certainty that it never saw "Baz" because it never saw anything hashing to "XY" and so "Baz" could not have been seen either.