I am trying to summon up my Philosophy Logic Courses from a very long ago. I seem to recall that a Tautological Proof began with the assumption of a negative, then steps were taken in order to methodically prove the ‘negative.’
If the negative could be unproved, then the Proof was determined to be a positive. It the ‘positive’ could not be proved in that fashion then the negative was then ‘proved.’ I know that I no longer have those text books but I may still have my notes.
The source of the fallacy is the assumption that something is true unless proven otherwise. The person making a negative claim cannot logically prove nonexistence. And here's why: to know that a X does not exist would require a perfect knowledge of all things (omniscience).