Classical rhetoricians listed five canons of rhetoric: invention, arrangement, memory, style, and delivery. For them, "invention" meant finding the arguments that would do the best job of persuading a given audience. In other words, it meant "inventing" an argument by finding the best means of persuasion. In our modern terms, invention means something like brainstorming, the first stage of the writing process. The pages listed below explain what invention is and how to come upon the best ways to invent a document.

• Silvae Rhetoricae
  Brigham Young University
  
  
• Inventio
  Wikipedia
  
  
• "Cicero on Invention"
  John F. Tinkler's Homepage at Townson University
  
  
• Planning (Invention)
  Purdue University Online Writing Lab

For more on classical rhetoric, see the following:

• What is Classical Rhetoric
  Martin Cothran at Memoria Press
  
  
• Classical Rhetoric
  DePaul University