A deductive argument in common use that is valid but unsound?

Question

A deductive argument in common use that is valid but unsound? What kind of premises can we be certain are true? What kinds of deductive argument may never be sound because their premises cannot be proven as true?

Answer 1

If people give out poisoned candy at Halloween, then trick-or-treating is dangerous.

People give out poisoned candy at Halloween, therefore trick-or-treating is dangerous.

The inference is valid. If people are handing out poison, don't let your kid take it. However, the fact is unsound. There has never been a single documented case of poisoned Halloween candy.

Hope that helps.