Last year, I developed an introduction to threshold concepts for the Higher Education Academy. It is now available online: https://www.heacademy.ac.uk/knowledge-hub/reflecting-threshold-concepts-introductory-tool Let me know what you think. I am always happy to talk about the applications of threshold concepts in education and for development in business settings. Contact me for more information.
Formal systems, studied by logicians, mathematicians, philosophers and others, are (usually) characterised by a specified and restricted set of symbols, rules for making statements from those symbols, plus rules for generating new statements. So from these basic rules and instructions, statements can be generated that are regarded as products of the system; they are called theorems.