Arguments in everyday life are rarely completely explicit. They usually de- pend on unstated assumptions that are understood by those involved in the conversation. Thus, if we are told that Chester Arthur was a president of the United States, we have a right to conclude a great many things about him— for example, that at the time he was president, he was a live human being. Appeals to facts of this kind lie behind the following argument:
Benjamin Franklin could not have been our second president, because he died before the second election was held.
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This argument obviously turns on a question of fact: Did Franklin die before the second presidential election was held? (He did.) The argument would not be sound if this explicit premise were not true. But the argument also depends on a more general principle that ties the premise and conclu- sion together:
The dead cannot be president.
This new premise is needed to make the argument valid in the technical sense.
This new premise is also needed to explain why the premise supports the conclusion. You could have made the original argument valid simply by adding this:
If Franklin died before the second election was held, then he could not have been our second president.