http://www.infidels.org/news/atheism/logic.html#circulus
Circulus in demonstrando
This fallacy occurs if you assume as a premise the conclusion which you
wish to reach. Often, the proposition is rephrased so that the fallacy
appears to be a valid argument. For example:
"Homosexuals must not be allowed to hold government office.
Hence any government official who is revealed to be a homosexual
will lose his job. Therefore homosexuals will do anything to hide
their secret, and will be open to blackmail. Therefore homosexuals
cannot be allowed to hold government office."
Note that the argument is entirely circular; the premise is the same as the
conclusion. An argument like the above has actually been cited as the reason
for the British Secret Services' official ban on homosexual employees. Another
example is the classic:
"We know that God exists because the Bible tells us so. And we
know that the Bible is true because it is the word of God."
Circular arguments are surprisingly common, unfortunately. If you've already
reached a particular conclusion once, it's easy to accidentally make it an
assertion when explaining your reasoning to someone else.
-- David McFadzean david@lucifer.com Memetic Engineer http://www.lucifer.com/~david/ Church of Virus http://www.lucifer.com/virus/