Historians may desire to investigate an event using a primary source if (d) it "describes the event" as it was experienced at the time. A primary source is a first-hand account of an event or period. It can be an original document such as a letter, a photograph, or a map; or it can be a secondary source such as an encyclopedia article or historical novel. Using only a secondary source to learn about an event can be very useful, but it cannot give an accurate picture of what really happened because the writer of the secondary source did not participate in the event. For example, if we wanted to know how President Lincoln felt about slavery before he made his famous Emancipation Proclamation, we could read his letters written from 1837 to 1860. These letters would be primary sources used by historians to understand Lincoln's views on slavery. Similarly, if we wanted to know more about the Vietnam War, we could read articles written by participants or observers of the war. These articles are called secondary sources because they were not written by someone who took part in the events themselves. Authors of these articles included journalists, activists, and politicians who lived after the war and wrote about their experiences.