How do you exactly cope with the stupidity of others?

What are some good ways to cope with for instance stupid family members or friends? (Assuming that leaving them is not a viable option, although tempting).

It's impossible to have a discussion about abstract concepts and their applicability with my girlfriend and I get so frustrated. She can't discern between hypothetical and reality. If I set up a hypothetical to talk about a concept etc, she will basically reason out that because it doesn't exist in our world, then it can't exist.

I'm fuming on the inside everytime I can reason something out, which she agrees to, only then disagree two sentences later.

Please help - I will end up a lonely bitter person, if I don't learn to cope with it.