Interesting concept: In the 1961 Robert Heinlein novel, Stranger in a Strange World, there are "Fair Witnesses", people who have been trained and certified to tell the truth, the whole truth and nothing but the truth. They are the last word in court cases, etc. They cannot lie.
An example given was that if you asked a Fair Witness what color a house was, he'd answer "white" after walking around and verifying that all four walls were white. However, if he walked away and you asked him later what color the house was, he wouldn't swear it was white, because someone might have painted it since he last saw it.
Other than that, "true" is in the eye of the beholder. Children (and too many adults) have the ability to convince themselves that something is "true".
That reminds me of a cute little story: A little boy ran into the house and breathlessly told him mom that there was a lion in the backyard. "A lion?", asked his mother, "Really? A lion? Don't you think it's just the neighbor's big dog?"
"No, really, Mom, it's a lion!"
The mother says, "Now, we've talked about lying and how you need to tell the truth at all times. I want you to go upstairs to your room, think about this a while and talk to God about telling the truth."
The little boy goes upstairs and comes down a little while later.
"Did you have a talk with God?" asked the mother.
"Yes I did. And He said He thought it was a lion at first, too!"