Thesis : The play Death of a Salesman demonstrates that trying to live up to conventional expectations in a capitalist society is likely to result in mental illness and tragedy.Your research paper needs only one outside source that is, a source in addition to the primary source. So if you decide to write about The death of a salesman, your primary source is the death of a salesman, and your secondary source would be an article in a scholarly journal on some aspect of the death of a salesman.Your secondary source can help you to support your thesis, or it might be something you disagree with. The purpose of using a secondary source is to show that you can join a conversation about your chosen text.