Actually seeing is not always reliable, or feeling. Hallucinations, for example. Reading a book on the brain, some of those by Oliver Sacks are really good, can make you doubt your ability to experience reality as something really real. Everything passes through our brain filter, and sometimes gets a little distorted, even for people who entirely sane and with normal brains.
It gets even worse than that. Memories, after a period of time, can become "contaminated" by other memories, or by our own desires, or by outside influences. What you remember may not even have happened the way you remember it.
"A casual stroll through the lunatic asylum shows that faith does not prove anything." - Friedrich Nietzsche