Why do you believe that "not everyone can understand chemistry"?
Perhaps I should be a little more specific. When I say "everyone" below, I mean "the overwhelming majority of people in the average range of human intelligence, without debilitating mental deficiencies". This group spans virtually everyone you are likely to encounter, from car mechanics to school teachers, to postal workers, to professional software engineers.
People have blind spots, but in general they're pretty smart when it comes to the things they think about every day. I believe:
* Everyone has the mental capacity to learn $RANDOM_DISCIPLINE if they put in the time and effort. A good teacher can dramatically accelerate the process, while a bad teacher can bring the process to a grinding halt.
* Everyone has a limit to how well they can understand $RANDOM_DISCIPLINE, (even Terrence Tao) but most careers do not demand that workers reach that limit. One can be a successful researcher without performing at the same level as Terrence Tao. One can be a successful software engineer without being a computer science researcher. One can understand advanced topics in math or computer science or chemistry without having personally discovered them. One can work as a chemistry lab technician, or a high-school chemistry teacher without winning a Nobel prize.
You can replace $RANDOM_DISCIPLINE with things like "computer science", "constitutional law", "how to reassemble a car", "chemistry", etc.. If you start getting very specific, you might be able to get me to disagree with statements like "Not everybody has the mental capacity to have invented Inter-universal Teichmuller Theory".