What is mendacity?

One of my favorite movies is 'Cat on a Hot Tin Roof'.  One of the themes of the movie is mendacity.  Mendacity is the tendency to lie. 
Mendacity is something we live in.
I once discussed with some people the need for white lies.  I may not like to lie but I do see the need for it in this world.  Most of the people do not want to hear the truth.  They prefer to be lied to eventhough they say they do not want to be lied to. 

The question is "when is it necessary to lie?"  Do we have to lie to keep a job?  Do we have to lie to protect someone else?  It is difficult to decide.  One of the commandments says 'Thou shall not bear false witness against your neighbor'.  It is difficult to decide when it is the right thing to do or not. 

Lying is something that many of us do on a regular basis especially if you work in the legal system.  But is it lying if you tell someone something and they take the words you have said and change the meaning of the words to fit them.  Who knows if it is lying or not?  I sure do not know.  I just try not to lie and pray that if I lie that the Lord will forgive me.