She writes on the blackboard the single sentence: Everything written on the board is false.
What are we to do when people disagree with one another? Is it possible for one argument to be right while another is wrong? Or is everything just a matter of opinion?
In some cases, there is a way to tell good arguments from bad using what is called informal logic. This name distinguishes it from formal logic, which is used in mathematics; natural language is less precise than mathematics, and does not always follow the same rules. (…)
The distinction between validity and truth is important. Technically, logic cannot establish truth; logic can only establish validity. Validity is still useful. If an argument is valid and the premises are true, then the conclusion must also be true. Once we know that an argument is valid, the premises are the only possible source of disagreement.
photo { Bob O’Connor }
