The important part is at the end of the article: "<i>For those interested in building a stronger foundation in logic, the Open Logic Project provides excellent free educational resources on propositional and predicate logic, formal proof systems, and other topics such as modal logic and set theory - all targetted towards a non-mathematical audience.</i>"<p><a href="https://openlogicproject.org/" rel="nofollow">https://openlogicproject.org/</a>