On 3/13/2012 2:07 PM, Warp wrote:
> John VanSickle<evi### [at] kosherhotmailcom>  wrote:
>> The answer to your dilemma is that invalid arguments prove *nothing*.
>
>    I appreciate your input, but it's not what I was asking. I don't have
> a dilemma. I'm looking for a definition.

I would venture to say that relevance is the key criterion to consider 
when deciding whether given observation is evidence for or against a 
given proposition.  In fact, many of the logical fallacies that we are 
warned about (ad hominem, ad populum, etc.) in courses on formal logic 
are called Fallacies of Relevance.

For an observation to be valid or proper evidence for a given 
proposition (or against it), there must be a *necessary* relationship 
between the observation and the proposition.  If there is no such 
relationship, then we can conclude that the observation is not valid 
evidence for or against the proposition being considered; and really, to 
say that a given observation is not valid evidence is to say that it's 
not evidence at all.

Now for observation A and conclusion B, there are four possible 
relationships between them, each of which can be stated in two different 
ways which assert the same thing:

"If A, then B," or, "If ~B, then ~A." (A & ~B = false)
"If A, then ~B," or, "If B, then ~A." (A & B = false)
"If ~A, then B," or, "If ~B, then A." (~A & ~B = false)
"If ~A, then ~B," or, "If B, then A." (~A & B = false)

If we cannot affirm any of these four possible relationships, then A and 
B are irrelevant to each other, and A is not evidence for or against B. 
  But if we can show that at least one of these four conditions is true, 
then A is valid evidence on the question of B.

This probably sounds a bit tautological, but that's the best I can do at 
the moment.

Regards,
John

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