How do you know what you know?

I believe that I know what I know through my experiences and from the present.  I can be sure of what happened in the past because it happened to me and my mind has the capability to remember things.  I can be sure of the present because the present is happening right now; and I can be sure of things that are happening as we speak because I have all of my senses.  I think the only thing we can’t be sure of is what we hear.  I say this because somebody can say anything they want to say but that doesn’t mean it’s true.  You also don’t know what state that person is who told you any kind of information or facts.  You can believe half of what you see only because you’re using your sense of sight to make a judgment.  On the other hand, even if you see something that doesn’t mean that what you see is actually what it seems.  One thing you can be sure of is your existence.  I think that’s the only thing you actually are sure of.  I say this because you can be certain that when things are happening to you that they’re actually happening.  The only people in this world that might not have an assurance of what they know are the people that are mentally ill or the elderly.  The people who are mentally ill sometimes don’t have a grasp of that is going on around them and the elderly lose memory once they get a certain age.