Using Secondary Sources
Even if your teacher does not require that you use a specific number of "outside sources," you should know that your paper's authority will be enhanced and its thesis will appear stronger if you are able to demonstrate that you can use it to engage the field of scholarly discourse that predates your decision to write the paper. If you mention previous scholars' work, the social effect is like walking into a room full of people who are engaged in a spirited discussion and demonstrating that you can listen to what is being said and that you can find a way to connect what you have to say with the on-going discussion. You need not agree with your sources. There can be good reasons why you see things differently, and if you explain them, you may be able to raise doubts about even the most venerable published scholarship. Try your thinking with your teacher and get some idea of whether your reasoning is persuasive.
The sources also need not address your main text directly--an article on one of Marie's Breton lais might easily make claims useful to a developing paper on another of the lais, or on one of the Middle English lais. An article on the possibility of an incest reference in the third book of the Troilus might well yield important insights useful to a discussion of incest in "Degare" or "Emare." Studies of actual incest prosecutions in the medieval period might help determine whether the work's portrayal of incest was "normal" for the era, an outrageous subversion of the norm, or even a satire on the norm. When literary technique is at issue, even a study of unreliable narrators in modern fiction which you might have read in English 215 could suggest a method of analysis which might work on one of Chaucer's dream visions (wherein the narrator is loosely related to Chaucer-the-poet and potentially motivated by Chaucer's professional ambitions or more personal desires). The trick in using secondary critical sources amounts to thinking creatively about what you find.
For more advice about when and why scholars use secondary sources, click here.