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SUBJECT:  Validity
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At the risk of boring you, let me say a few more words about
explaining why an argument is valid.

Remember the sequence of possible *correct* answers in my last message
(here augmented with one more):

An argument is valid because:

	 * it is valid.

	 * the conclusion follows logically

 	 * "one premise leads you to the next,
	   and all premises combined lead to the conclusion"

	 * any situation that makes all the premises true
           also makes the conclusion true
	 
The first is true but useless as an explanation.
The second says a bit more, but leaves open what "follows logically" means.
The third says a bit more, but leaves open what "leads to" means.
The fourth says a lot more, giving one of the formal definitions.

But all fail as explanations of why a particular argument is valid,
such as PP4's argument from the first 3 premises to conclusion 4.

There's a missing premise, namely, a statement explaining why those
particular premises validly support that particular conclusion.  

And that kind of premise has to talk about the subject matter of the
argument, not just repeat a definition.