  Diodorus was a pioneering logician, and the most celebrated member of the Dialectical School of the 4th–3rd c. BCE.  His contributions to logic—in particular, definitions of modal terms and the criteria for a sound conditional—are covered in the article on the Dialectical School (see also Section 2 of the entry on    fatalism). The present article adds a conspectus of Diodorus’s other ideas. His use of paradox is at least as prominent in our ancient sources about him as are those constructive contributions to logical theory.