The first part of what you said is absolute rubbish. If I saw a man that I knew shoot another man but there was no evidence except my word does that mean it is not definite? In the court it might not count as sufficient evidence but it is still definite.The only things that are definite are the things that you can prove with evidence. Everyone's memory is faulty.
To quote a less successful Democratic presidential candidate, it is possible to be certain and wrong.