Quote from: sed on March 17, 2015, 06:35:16 PM

The old yarn about "not proving a negative" isn't in any way a QED it's a popularism which, while related to an actual fact of logic, is actually a fallacy in the way that you've deployed it.

In fact, "proving a negative", as you say, is one of the basic argumentation techniques for proof. You take a statement you want to prove, you negate it, you show that that leads to absurdity, this is QED for the original statement. The crucial point, in fact, relates to the square of opposition (cf. Arisitotle's Περὶ Ἑρμηνείας, Latin: De Interpretatione). Contradictories divide up the space between them with nothing left so that either A or ~A is true, it's impossible for it to be otherwise. When you're dealing with propositional/sentential negation then you are dealing with contradictories so it is quite useful to employ negation in proofs. On the other hand, contrary statements allow for the middle ground to also be true and this is where you have to be careful employing negation.

All cats are black [is contradictory to] Some cats are not black (= not(all cats are black)).
All cats are black [is contrary to] No cat is black.

In the contradictories, either the left or right is true, no other options (in fact, I believe in the real world that the right side is true, some cats are not black). However, in contraries, it's possible that neither the left or right is true (and, btw, I believe this is the case in the real world for this example).

The old yarn about "you can't prove a negative" is a popularism which is often employed without considering how vague it really is. If you really want to understand syllogistic logic and negation, start with the square of opposition.

Best!

Yes, I know all about proof by contradiction and reductio ad absurdum. But thanks for the boring refresher course.

Are you claiming that presumption is affirmative? Of course not; that would be stupid, and almost as pointless as niggling over paraphrasing.

So spare us the sophomoric pedantry and allow some leeway for rhetorical shorthand. Even Yahoo Answers knows better than to pull that crap:

Quote

https://answers.yahoo.com/question/index?qid=20110307170939AA7BPmy

It is short hand for a much more logical expression.
Which is, "You cannot prove a universal, existential negative."
In other words, you cannot prove that some hypothetical does not exist, anywhere in the universe, because that would require that you be able to look everywhere at the same moment. And, of course, if the hypothetical something, in question is claimed to be invisible and undetectable by any means, in principle, it gets even sillier to attempt to disprove that hypothetical's existence.

But saying all that, over and over gets really tiring, so most people just shorten it to, "You cannot prove a negative." and go on to do something more productive with their time.

The point is that 60% pro-bloat votes isn't anywhere near the desired, much less required, consensus. And you don't get to count the agnostic/DGAF votes as 'anti-anti-fork' as Gavin illogically did.