Fact or Fiction?

If I tell you a lie and I convince myself it's truth and I believe in it's truth. And you believe in it's truth. Is it still a lie? How can a lie be something true. I'm taking a fiction workshop so I find myself creating these stories and making them into "truths." This class is definitely helping me expand my creativity and drain my imagination for all it has to offer. If a made up story can be loaded with truths, can't a lie? And if that's the case....why is it so bad....especially if you never find out it's a lie? hmmm....just a thought. Any comments....?