Two negatives make NOTHING!

26 Aug

         No boys like me

                                        Some of my best friends don’t like me

                                        Tf, my life is awful

What’s wrong with this argument?

-besides being the lament of a ‘too-introspective’ teen girl

-besides consisting of more than 3 terms (violates Rule # 1 of the Valid Argument test)

Here’s what’s wrong – You can’t draw ANY conclusion WHATSOEVER from 2 negative premises.

That’s Rule # 5  for “Evaluating the validity of a syllogism” in a nutshell.

Rules 1 to 4 have focused on

  • the number of terms in a syllogism
  • the occurrences of each term (Major, Minor and Middle) in a proper syllogism
  • the ‘distribution’ of each term, that is –  the reach or how many ‘members in a set’ to which each term applies

With Rule # 5 (there are seven in total), we look now at what is called the QUALITY of  the premises in the syllogism.  Quality refers to whether a proposition is affirmative or negative.

Both common sense AND logic inform us that you can’t get ANYTHING positive out of a negative.  And if you can, then there is more ‘back’ argument that needs to be flushed out (unarticulated pre-suppositions or other propositions).

Imagine someone stepping outside of his office cubicle and shouting seemingly à propos of nothing…..

-the picnic is not going to happen!   

-there is no pizza in the freezer!

– therefore, I’m happy

We’d conclude that this guy was nutty!

So what do we do when we run across an argument in a letter to the editor that is drawing an affirming conclusion from a bunch of negative ‘facts’ – that is, when their premises are either E or O statements?     

(E – No pizza is in the freezer; 0 – Some picnics are not going to happen)

First of all, since we are equipped with logic as a tool, we know to ask for more information.  Their claim, constructed from 2 negative premises, can’t stand on its own.  There HAVE to be affirmative propositions (A  – All food is what makes me happy; I – Some meals are better than no meals).

Don’t be afraid to gently push back against an argument-maker by asking questions.  After all, the burden of proof is on him who makes the claim.

And by the way, this is an easy way out of an argument you might not want to tackle.  If you can point out gently that someone is basing their argument on negative premises, you don’t even HAVE to consider the conclusion – it’s irrational to begin with!

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