Wikipedia:Verifiability
This is an attempt to summarise an existing guideline that we've followed for a while but not written down. Mind you, in writing it down, we may decide it's rubbish!
The goal of Wikipedia is to become a complete, accurate encyclopedia.
We can't be sure of our accuracy if we include information which cannot be verified.
Therefore, include nothing that you cannot verify.
If you try to verify some information in an article, and cannot, raise the problem on the talk page (you may wish to temporarilly remove the information). Someone else may have additional resources and be able to verify it.
You should make it easy for people to verify your information by citing your sources. They don't have to be online - books, newspapers, etc - all good.
Verifiability is one problem with articles on obscure subjects. By concentrating onf verifiable subjects, we also concentrate on important subjects.