Given:
A = [ Aaron features ],
B = [Bob features],
X = [all countries in Europe ]
What must be true if:
|A ∩ B ∩ X| = 1
Even though I don't see the relation between cardinality of sets, I kind of understand this set operation by doing a Venn diagram.
In the end what must be true for that operation logically speaking?
If $|A\cap B\cap X|=1$, then there is exactly one element that is in all three sets. That is, there is exactly one country in Europe that is among Aaron's features and also among Bob's features.
This seems absurd, since countries are not "features" of a person, so it would seem that $|A\cap X|=|B\cap X|=0$ (and hence $|A\cap B\cap X|=0$). I would consider "features" to be things like eye color, name, email address. But perhaps "features" can include countries somehow, by redefining that word from its usual meaning.