Assume good faith
Assuming good faith (AGF) is a fundamental principle on Wikipedia. It is the assumption that editors' edits and comments are made in good faith. Most people try to help the project, not hurt it. If this were untrue, a project like Wikipedia would be doomed from the beginning. This guideline does not require that editors continue to assume good faith in the presence of obvious evidence to the contrary (e.g. vandalism). Assuming good faith does not prohibit discussion and criticism. Rather, editors should not attribute the actions being criticized to malice unless there is specific evidence of such.
When disagreement occurs, try to the best of your ability to explain and resolve the problem, not cause more conflict, and so give others the opportunity to reply in kind. Consider whether a dispute stems from different perspectives, and look for ways to reach consensus.
When doubt is cast on good faith, continue to assume good faith yourself when possible. Be civil and follow dispute resolution procedures, rather than attacking editors or edit-warring with them. If you wish to express doubts about the conduct of fellow Wikipedians, please substantiate those doubts with specific diffs and other relevant evidence, so that people can understand the basis for your concerns. Although bad conduct may seem to be due to bad faith, it is usually best to address the conduct without mentioning motives, which might exacerbate resentments all around.
Be careful about citing this principle too aggressively. Just as one can incorrectly judge that another is acting in bad faith, so too can one mistakenly conclude that bad faith is being assumed; exhortations to "Assume Good Faith" can themselves reflect negative assumptions about others.
Everyone makes mistakes, both behavioral (such as personal attacks) and content-based (such as adding original research). Most of the time, we can correct such mistakes with simple reminders. However, there will be disagreements on Wikipedia for which no policy or guideline has an easy answer. When disagreements happen, ill intent may not be involved. Keep a cool head, and consider dispute resolution if disagreements seem intractable; many of them are not.
Violation of policies—such as engaging in sock-puppetry, violating consensus, and so on—may be perpetrated in either good or bad faith. There are processes for dealing with all of these, and sanctions for repeated violation of policy will apply regardless of whether bad faith was involved or not.