A primary historical source is any piece of information that was created by someone who witnessed first hand or was part of the historical events that are being described.
The most common primary sources are journal entries or letters.
As a general rule, modern historians prefer to go back to primary sources, if available, as well as seeking new ones. A work on history is not likely to be taken seriously if it only cites secondary sources. This of course does not preclude secondary sources being used as a guide to find and interpret primary sources.
