Say everything you mean, or mean everything you say?

Tweet

There is a saying, "mean everything you say, don't say everything you mean". What do you think? Do I always have to tell the truth, or is it enough with always speaking the truth when I am saying something? Some times telling the truth can be painfull for some one else, maybe it is better to not say anything (and not deny it either, that means). But is this a lie or not, to not tell the whole truth?