Final answer:
Aristotle's idea of speech genres was formed based on his works in rhetoric, philosophy, theatre, particularly the emphasis on tragedy, and his comprehensive system of logic. His text, The Poetics, identifies essential theatrical elements and promotes the educational value of tragedy over comedy.
Step-by-step explanation:
Aristotle formed his idea of speech genres primarily through his work in various disciplines, including rhetoric, literature, and drama. In his writings, he attempted to define what works fall into the category of literature and established criteria that would influence the Western canon for centuries. Aristotle's Rhetoric is foundational in establishing speech genres as it looks into the art of persuasion and complements his works in philosophy and politics. His analysis of theatre in The Poetics suggests that for theatre to be effective, it must provide both spectacle and educational value; however, he only considered tragedy as a valid genre, disregarding comedy due to its perceived lack of educational content.
A significant influence on Aristotle's categorization of speech genres was also his doctrine of the four causes, which sought to explain the nature of everything by addressing material, formal, efficient, and final causes. Aristotle's contributions to logic, with his complex system that underlined dialectical arguments, also played a role in how he perceived different forms of speech. This comprehensive approach, grounded in dispassionate reason and a quest for understanding the universe, heavily contrasted the Sophists' appeal to emotions in their rhetoric.