True Story: Clinton wasn't totally crazy when he said that.
The line of questioning was:
That's when Clinton uttered the infamous "depends on the meaning of 'is'" line with respect to the statement that there "is no sexual relationship."
Clinton's argument about the word "is" relates to time-frame. At the time that the statement was made that there is no sexual relationship it was true because, at that point in time no sexual relationship existed. The relationship had ended. Clinton's argument is about the difference between "is" and "was."
It's been portrayed as a big semantics game for obvious reasons, but remember that Kenneth Starr was trying to go after Clinton for perjury. Clinton's argument about why what he said was true, and therefore not perjury, is actually pretty valid.
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