mindseye: Modus tollens is a form of structured argument -- sometimes called proving the contrapositive -- where the advocate shows that the conclusion has failed, and therefore the hypotheses could not have been satisfied.
Symbolically, it has the form:
If p is true, then q is true.
q is false.
Therefore, p must also be false.
Although modus tollens is a logically taut argument style, it fails if the implication statement ('if p is true, then q is true') is unjustifiable.
Example:
If censorship is bad, then I have tentacles growing out of my head.
I do not have tentacles growing out of my head.
Therefore, censorship is good.
The argument fails because the 'if' statement is not convincingly true.
Hope that helps.