So, if it was based on concrete evidence like you suggest, then why do so few actually get it? This is my point, and my proof. It isn't based on concrete evidence at all. It isn't simply an extension of our senses. It is an altogether new sense. I never claimed that students reach a mostly formal state of thinking by 8. I stated that by the end of algebra 2, the transformation is mostly done. And I base that on performance, not appreciation.