The growth of human knowledge is, in many ways, like a great conversation. Each new generation of scholars critiques the opinions of older scholars, sometimes defending and sometimes offering objections, much as people do who are having a polite but serious conversation on important matters. In a normal conversation it is easy to know who is talking, and it is easy to keep track of who has offered which opinions. In the Great Conversation of human knowledge this is more difficult, since the conversation takes place over many centuries, and in many languages.

Citation could be thought of as a set of conventions that have been adopted to help the Great Conversation proceed more smoothly. Any conversation has such conventions. For instance, it is impolite to walk up to a conversation and start speaking before you know what the conversation is about. In the Great Conversation, a new speaker is also expected to know what the conversation is about before offering his or her own opinion. To show that you know what has been said by others, you need to explain how your own opinion has been influenced by previous “speakers” in the Great Conversation. To do this, you need to refer to those speakers, i.e. you need to cite their work.

When to give credit
(adapted from Avoiding Plagiarism by Purdue University’s Online Writing Lab)

Need to cite when	No need to cite when
referring to someone else’s ideas, opinions, or theories, such as by paraphrasing copying exact words reprinting or copying graphical elements such as diagrams, illustrations, maps, charts, and pictures using ideas from others given in conversation, interviews, correspondence (letters or email) or heard during lectures, speeches, and from media such as television and radio	using ideas, opinions, or theories that are genuinely original with you writing up your own experiment results including your own artwork or other original creation recording anecdotes about other people, in which those people remain anonymous using common knowledge according to accepted criteria

Purdue University Online Writing Lab, Avoiding Plagiarism: Safe Practices, 18 September 2007, <http://owl.english.purdue.edu/owl/resource/589/03/> (05 February 2008).

Citations in the How to Credit Sources section use Chicago style.