[lisper]

That depends on the claim. For example, I can easily prove the (negative) claim that Bigfoot is not in Times Square. The proof goes something like this:

1. If Bigfoot were in Times Square, someone would notice.

2. No one has noticed.

Therefore it cannot be the case that Bigfoot is in Times Square.

That proof works because of the particular properties of Times Square (e.g. that it's full of people 24x7) and doesn't apply to the forests of the Pacific Northwest.

[awptimus]

What if Bigfoot is dressed in a burka? Under your assumptions your system is logically sound and under the system you can conclude, but in broader context you assume that which you would like to prove. You started with no one saw bigfoot, and add it as a necessary condition for bigfoot's existence. It's not a necessary condition.

If your assumptions are false, then so is your conclusion, in the end.

[lisper]

Bigfoot is seven feet tall so even if he were wearing a burka someone would notice.

Besides, Bigfoot is Jewish.