If I tell you that I am a liar, I create a perpetual logical rebound. If it is true, then it is false, so how can it be true? And if it is false and I am not a liar, then I am telling the truth, in which case I am lying. The undecidability in this predicament comes to rest only if the statement about myself and the moment of saying it can be separated in time, so that I am no longer a liar while I am saying so: 'sometimes I am a liar" and "I used to be a liar" make perfect logical sense because they separate the reliability of the narrator from the unreliability of the narrated, even when they are the same person. The pragmatic contradiction is resolved by splitting the T between past and present.