|This essay contains the advice or opinions of one or more Wikipedia contributors. Essays are not Wikipedia policies or guidelines. Some essays represent widespread norms; others only represent minority viewpoints.|
(This is not a policy or guideline - it is intended to explain why some policies and guidelines exist)
The goal of Wikipedia is to create an encyclopedia through the efforts of multiple, independent editors editing in a co-operative and collegial manner.
Encyclopedias are not places for philosophical discourse on matters of sex, religion, economics and politics, nor for presenting personal opinions about anyone or any topic. Wikipedia restricts articles to material previously written or discovered by others, and not written or discovered by editors.
To that end, Wikipedia has developed policies and guidelines - many of which are couched in less-than-obvious language. This essay seeks to explain some of the rationales as best it can.
From the beginning
WP:OR - No Original Research
Articles should be based on claims made by other people on published sources, not on assertions by Wikipedia editors. Wikipedia, as a community, has decided that claims should not be combined by editors in order to make a new claim not based on a prior secondary or teriary source. For that reason, the use of primary sources has been deprecated. (see WP:OR)
Editors working on an article should keep in mind the entire topic which the article covers, and avoid focusing too much on a single piece of it. (see An article is the sum of its parts)
Encyclopedias deal in facts and notable opinions. Facts should be derived from reliable sources. Opinions should be attributed to those persons or organizations holding them. Conflating opinions and facts is a problem which editors must avoid.
So what sources can we use?
WP:V - Verifiability
WP:V requires that an outsider be able to "verify" that the source says what the editor using it says it says. It also says that editors should use the best available sources. It does not say that such verifiability is the same as "truth."
Consider how carefully the source has been or may have been vetted by others, and to what extent responsible parties stand behind the source. Thus, major news organizations, which fact-check articles, are generally considered reliable sources for matters of facts relating to news. Scholarly books and articles are generally reliable sources for scholarly views. It is difficult to determine reliability in sources about current research, current controversies, current events and the like, but Wikipedia is not a newspaper and has no deadlines. Where any source has removed or edited content in a later edition, especially on matters of current events, the latest version is preferred to the prior version for statements of fact.
Matters of opinion can not be checked for correctness, and so should always be ascribed as opinions. They must still accord with Wikipedia policy on reliable sources.
Much of the Internet is "self-published". Also, self-publishing and "vanity press" printers will print books for people willing to pay the expenses of printing their books. Using self-published sources is problematic for claims of fact and caution is advised.
Private (e.g., personal) and group blogs are considered "self-published." Blogs under the editorial control of news organizations using their own staff are generally not considered self-published (see: WP:NEWSBLOG).
Wikipedia allows use of such sources only when the author is an "expert in the field", or where the person who is the subject of a BLP has a blog and the blog is used to present what he or she specifically claims about themselves. This may lead to debates about who is an expert and what the field is. It is wise to consider all SPS references deprecated. If the fact is sufficiently important, it will eventually be published in a better source. (See: There is no deadline.)
Articles of special concern
Articles relating to living persons and small groups of living persons (such as a family) are covered by the Biographies of living persons policy. All contentious claims about a living person are required to be well sourced. Unsourced contentious claims are required to be deleted. Opinions about the person must be ascribed to the person or group holding the opinion. In addition, opinions must not be presented as facts. Contentious opinions within a BLP are generally deprecated.
Articles which make "allegations" make bad encyclopedia articles, especially when any sort of POV can be attached thereto.
Articles relating to medical issues are subject to the guideline on Identifying reliable sources (medicine). The goal is to avoid confusing readers with claims presented as fact that are not borne out in specialized sources. This does not mean that such claims are excluded entirely from articles, but that they should not be presented as accepted facts. Claims presented as opinion and properly ascribed as such, may be included so long as they are not given excess weight, per Neutral point of view: Due and undue weight.
Other areas may also have specific guidelines on sources, and govern in those areas. The principle in each case is that readers should have accurate information, without having divergent views elided.
Conflict of interest
Any editor with a conflict of interest in a topic is advised not to edit it. If in doubt, it is better to suggest edits on the article talk page than to improperly edit the article. Conflicts may arise from personal financial interest in a topic, employment by any entity with an interest in a topic, or any personal interests which do not allow for editing the topic with a neutral point of view.