Boethius

Although Boethius saw himself as a philosopher, he was a source of classical learning for medieval scholars, and his work reflects the "treatise of logic" (486, RT). In this passage Boethius titled, "An Overview of the Structure of Rhetoric", he begins by outlining the three species of rhetoric which are judicial, demonstrative, and deliberative. Subjects debated are largely of civil importance to cultures - and are shaped by the species they are categorized into.
     "when it seeks the ends of justice in a court of law, it becomes judicial; when it asks in an assembly what is useful or proper, then it is a deliberative act; and when it proclaims publicly what is good, the civil question becomes demonstrative rhetoric (489)."
This quotation explains this principle and how the type of argument and its kairos are determined by the species it's categorized into.
Boethius goes on to comment on the importance of rhetorics 5 parts (invention disposition, style, memory, delivery) and how a true rhetorician must utilize all these parts or they will be missing an important element of direction in their argument.