Unnecessary and Necessary Lies

Sometimes I wonder, "Is there such a thing as an necessary lie?"

Most parents and teachers I know say that it is never good to lie. But when you start thinking about this concept you realize that maybe a lie could be useful. For example, If you were fighting in a war and got held as a prisoner, to escape the country you would need to lie.

So I suppose that another question this question raises is, "If there are such things as unnecessary and necessary lies, then when would the lie be considered necessary or unnecessary?"

I think that this is a very interesting topic.