These two sources are further broken down into formal and informal sources. Internal information is what you own. It might be expert or nonsense, but you believe it to be reliable and valid or you would not persist with it. Most importantly, however, internal information is broadly defined. It includes all of your factual and expert knowledge and experience as well as your nonrational biases, prejudices, values, beliefs, and feelings.
External information comes from two sources, formal and informal.
Some people live by this source and seem to get by. They are not bothered by concerns of validity and reliability. Formal sources include expert information whether located in person or through publications. Your level of confidence is raised when you find information in print. The assumption is that writers submit their writing to public examination.