How can I critique a historical source?

One of the most important skills of any historian is looking beyond a historical source, be it a poster, a letter, or a diary, and being able to place it in a wider context and therefore understand it more. To do this, consider its 'provenance', that is, nature, origin and purpose. Nature considers WHAT, what is the source? A diary, a poster? What does this mean? Most importantly, who is seeing the source? A diary is private so may be truthful, whereas a poster for public display wants a certain message to come across. Origin considers WHO wrote it, WHERE they wrote it, and WHEN they wrote it. When looking at the question, consider relevance of the origin. Is it in a relevant time period, by a relevant figure? Further, what kind of bias could be held by someone from that place, or of that position?Purpose considers WHY, an absolutely fundamental question for any source. Why does it exist? Why did someone produce it? Using your discoveries from nature and origin, unravel why the source is there. If it is to spread propaganda, then it is unreliable as it is painting a false picture. If as a personal record to look back on, it could well be more reliable. With this, you can critique the reliability and utility of a source. Remember, a source such as an official document could be unreliable if it is used for propaganda purposes, but could be useful when looking at what kind of information is spread by that government, or what they want citizens to believe.