Wikipedia:Saying something doesn't make it so
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On Wikipedia, at least, simply saying something doesn't make it so. This applies in disagreements and arguments, both to content and sources. Asserting something without explanation or demonstration is known as an empty assertion.
IRL, we can often see that saying something doesn't make it so: no matter how much an adult wishes to be six foot five (or five foot six), one is stuck with the height one is at. However, in both IRL and Wikipedia, when conversations or conflicts arise editors may often argue through empty assertions, thus making demands without reasons and ordering without persuading. Everyone is familiar with empty assertions, the classic being the one word sentence, "Because." This may be fine coming from a toddler, but coming from someone engaged in a collaborative project to make and make better an encyclopedia, this is unacceptable.
In regards to content, no matter how much one believes something, Wikipedia requires verifiable sources. See WP:V and WP:C. We are not documenting our personal beliefs, but those published in sources. Most likely, if you as an individual believe something, you can find a source which believes the same, or which documents others having that belief. This is much more convincing than simply repeating an assertion over and over.
In regards to sources, no matter how much one believes something, if another editor or editors add cited content which disagrees then you have two options. You may explore the source and demonstrate, if this is the case, that it is not a verifiable or reliable source. This will probably not be an option in most cases. The second, and most likely, option is to find a source demonstrating the opposite, your belief, or demonstrating that people hold that belief.
In regards to content and sources, the least effective thing one can do is to simply assert over and over, "X is/isn't true," or "X is/isn't a reliable source." Empty assertions, are never convincing. These statements do not make themselves true, no matter how many times you say them.
Empty assertions usually result in escalation of conflicts. When someone asks that another person do something but is unwilling to give you any reason why that person should do it, this is unconvincing, thus often unproductive. When someone demands that another does something, but is unwilling to give you any reason why that person should do it, this amounts to bullying or an attempt at dictatorial control. One is far more likely to convince other users by convincing them, rather than demanding.