We need to understand four kinds of knowledge in order to talk about the appropriate authority a writer can claim. Personal knowledge is what you have yourself experienced or witnessed directly. You have a right to claim authority to say what you have seen or experienced no matter who believes something else about it. In writing, however, you should make sure you SAY you are speaking from personal observation or experience. Personal knowledge has many advantages, but it also has some disadvantages in persuasion which the writer should be aware of.
Common knowledge is the ordinary facts of existence which can be gathered from many sources.
There is a high degree of agreement among people about matters of common knowledge. The kinds of facts you can find in dictionaries, almanacs, encyclopedias, newspapers, and news magazines are regarded as common knowledge even if lots of people don't happen to know these things well. Much of what your teachers tell you and what you read in introductory level textbooks can be regarded as common knowledge as well because millions of educated people know these things and you can find them out from many sources, not just your textbooks or your teacher's lectures. Once you are sure you know something which you have good reason to know is common knowledge, you have just as much right to explain it to others as anybody else. On the other hand, if you have just learned something from primarily one source and have not had a chance to discover that it is contained in many sources, treat it as specialized knowledge until you are sure.