There are five canons of rhetoric: invention, delivery, arrangement, style and message. These categories provide a template for both analysis and creation of rhetorical works. They also offer a way to learn rhetoric.
A famous example of rhetoric is Martin Luther King, Jr.'s "I Have a Dream Speech." Although the subject matter of King's speech was not necessarily new, he ordered and delivered that subject matter in a style suitable to his large, diverse audience-- both a huge crowd in Washington, D.C., and Americans watching on television.
Modern studies of rhetoric address a more diverse set of concerns than did ancient studies. Classical rhetoric, and rhetoric for many centuries afterward, trained students to effectively persuade audiences in public institutions. Modern rhetoricians investigate discourse--the structures of language, communication and meaning in culture.
In rhetoric, invention relates to logos--the appeal of thought. From the Latin "invenire," which means to find, invention in rhetoric concerns finding something to say. Rhetoricians found certain categories of thought, called "topics of invention," conventional sources for creating arguments.
In composition, invention is what an author says as opposed to how the author says it.
The topoi, or topics of invention, are basic types of relationships between ideas. Each topoi is a template for figuring out what to say about a particular subject. Greek philosopher Aristotle defined two types of topics of invention: "common" and "special."
Special topics of invention refer to the three branches of oratory (judicial, deliberative/legislative and epideitic/ceremonial). Writers using the judicial topoi concern themselves with a past act in order to ascertain justice or injustice. Lawyers and judges tend to specialize in the legislative topoi.
The deliberative topoi looks instead to the future, in order to deliberate an idea and decide if a potential action will be advantageous or disadvantageous. Senators, representatives and lobbyists often use the deliberative topoi.
The common topics of invention, as defined by Aristotle, include definition, division, comparison, relationship, circumstances and testimony. In the "relationship" topic of invention, a rhetorician might investigate cause and effect, or contradictions between things as a way to frame an argument.
Many thinkers outside the field of rhetoric use the rhetorical topics of invention. In art history, for example, scholars often compare one artwork to another as a way of learning more about each object.
Invention in composition is basically a starting point. For example, a fiction writer may focus on the plot of a story and less on characters, setting or theme. Other writers avoid a fast-paced plot in order to set a particular mood or investigate a character. For example, Marcel Proust's "In Search of Lost Time" gives little plot but rather spends pages on the narrator's memories, settings and moods.