You ever see a dog that’s got its leash tangled the long way round a table leg, and it just cannot grasp what the problem is or how to fix it? It can see all the components laid out in front of it, but it’s never going to make the connection.
Obviously some dog breeds are smarter than others, ditto individual dogs - but you get the concept.
Is there an equivalent for humans? What ridiculously simple concept would have aliens facetentacling as they see us stumble around and utterly fail to reason about it?
My take:
Most things (especially abstract ones) that exists beyond the scope of the small-hunter-gatherer-tribe setup our brain is developed for: Quantum mechanics, climate change, racism, relativity, spherical earth, …
What separates us from the dogs is that we’ve developed abstract analytical tools (language, stories, mathematics, the scientific method,…) that allow us to infer the existence of those things and, eventually try to predict, model and manipulate them.
But we don’t “grasp” them as we’d grasp a tangled leash, which is why it is even possible for medically sane people to doubt them.
I’d argue that you can even flip this around into a definition:
If a person with no medical mental deficiencies can honestly deny a fact (as in: without consciously lying), then that fact is either actually wrong, or it falls into the “tangled leash” category.