One should verify what one knows. We all see the world though our own eyes. We all learn what we are taught; by teachers with their own eyes, who were taught by other teachers. Sometimes our trusted sources, or their sources, are wrong. Sometimes we forget. We should check our facts.
This isn't always easy. Sometimes it simply isn't practical to verify the Earth is round. In these cases, one should remember and convey their information as merely accepted.
When I worked at McD's, my first real job, the general manager often said this; I don't even remember why. I have learn since, if one is responsible for something, one better verify it personally.