It is always so enlightening to talk to people from a diffenet background than yours. In such a talk I heard the sentence "Never try to teach a pig to sing. It wastes your time and annoys the pig." Apart from being taken out of the context, for my own believes, it is a paradoxiacl statement because it is both true and false.
I guess that most people know why it is true. Then why is it false? In my practice in teaching, I have never seen anyone who cannot learn something if the person wants to. It may take time and dedication from both the teacher and the student but finally, if it is explained well enough, it makes sense. Here comes another realm which I will write about later, and that is instructionist vs. constructionist teaching.
Anyway, in short, you can always teach a person any knowledge and as long as the person is willing to accept and understand it, there will be a learning process. Of course, there will be people, who will disagree. I have heard numerous times critics to my observations who say that there are people who simply cannot get some sort of knowledge. Take math for example and another quote from the same book: " ". To my experience, since I have also taught mathematics, students who are not good at math are those who are afraid of it. I have had many university students who simply block when they see some math being written. However, if you sit with them and you work with them on their fear on first place and their lack of pieces of basic knowledge, after some time those students perform increasingly well.
So my belief and observation is that there are no types of knowledge who are especially reserved for "the more intelligent people". Mathematics is not more difficult than music but it is oftentimes incorrectly taught. To be precise, music is not well-taught either but it is considered less important than math and does not consitute such a great portion of one's education.
All in all, you can always teach anyone anything if the person wants to be taught. Then why do I agree with the statement? Because of the condition that the statement I made is true. If people are not interested in something, it is a good idea to let go. Alternatively, you may try to understand why they lack interest. Is it because people are scared of the subject (they have had bad experience with it), or that this doesn't really coinsides with their understanding of how their knowledge picture should look like and they really have no interest in learning the subject.
In short, it has been proven many times that humans are about equal in mental capacities with some small deviations from the mean but what distinguishes us are our interests and our socially accepted stereotypes that the people in some science shperes are more intelligent than other. Therefore, if a person is curious he/she must have a self-esteem, irrespective of what the person is curious about. In addition, everyone can self-develop no matter what sphere of science he/she has chosen.