Many of us enjoy imaginative speculation. It’s what has moved human society forward in thinking (brain evolution) and played a role in our survival. Our brain grants us many tools for imagination…dreams, patten seeking, stories - all communication. Its also plays a role in science (otherwise how would a hypothesis be introduced).
The thing is, reality (however you want to approach it) provides a benchmark to confirm shared reality. IF I am talking to someone, and a third person joins us, I can be pretty confident that the person I’m engaging with isn’t “in my head”. Our brains and senses can be fooled. It can be fun, or it can be disturbing 
How we process information through our OWN worldview drives our actions which in turn effect others and the world we live in. Many a person has limited themselves or chose very violent actions based on their perceptions that were in error.
Having a big pile of “I don’t know” or this idea is “interesting” is a helpful tool in categorizing which “beliefs” drive our decisions.
Personally, I dropped using the word “believe”, and use the word “confident”. It creates a process of weighing the story or claim against the evidence that the thing said is “close to true”.
Interesting that you question John Nash’s experience and life story (backed by evidence) YET choose to believe very wild unevidenced claims. Again the brain using its selective-bias.