I think the heart is the gut feelings which are generated by the limbic and primitive parts of the brain, and the head is the logical part of the brain. So, it is really that the different parts of the brain the contradict each others. The logical part, which we use for math and physics, requires more effort and energy, that is why most people are guided by their lower parts of the brain. Who is telling you that you cannot do it is the limbic system, but this is because your limbic system wasn't evolved to get you through the modern world. It was molded by natural selection to keep you alive in the wild in Africa between predators, which was a hostile environment. So, negativity was an advantage. Now it is not.