A snake hasn't any legs. Douglas Bader has no legs. Therefore a snake is a fighter pilot. What?

Question

Mu boy nigel is learning ratiocination (syllogism and all that) What's he doing wrong here?

Answer 1

Formal logic like this is, for the learner, best approached using Venn diagrams.

In a Venn diagram, a set is represented on paper by a circle or other closed loop. So draw a large circle (1) representing all no-legged objects. Snakes are no-legged objects, so a smaller circle (2) for snakes fits entirely inside circle (1): there are no snakes outside the set of no-legged objects. Now, another small circle (3) represents a set with only one member - the set of Douglas Bader. Since he has no legs, this must also fit inside circle (1) but not necessarily overlap or be contained in circle (2). Your diagram might look a bit like a face - a big circle with two "eyes" inside it. Sorry - it gets a bit tricky explaining this without drawing.

From the "eyes" diagram we can see that DB might not fit inside the snake category, and so the deduction is wrong. Of course, he might be a snake. You could draw the diagram to illustrate this possibility (2 concentric circles inside circle 1). This too is only a possibility. The point is that a syllogism is supposed to prove something, and if it can be false, it obviously hasn't been proved.

I hope this helps.

Answer 2

Douglas Bader had legs and arms, just his legs were made out of tin.

Not sure what examples to give you,

um heres one - a lightbulb emits light,
the sun emits light
therefore the sun is a lightbulb.

Its about taking two things that have something in common and trying to link them, it can end up being quite ridiculous though.
Its all very complicated but if you get the facts right then it works. Douglas Bader had legs, metal ones, but they were still his legs.

Answer 3

It plots nicely on a Venn diagram.

Douglas Bader appears in the set of things with no legs and the set of fighter pilots. But those two sets are not coincident or coterminal.
Some fighter pilots have legs, not all legless things are fighter pilots.
In fact, I suspect Douglas Bader is the only member of the subset formed by that double qualification.

Answer 4

He's forgetting about subsets. If A = B and B = C, then A = C only if they are absolutely equal to each other.

You remind me of the line from an Abbot and Costello joke which goes, "Just because all blueberries are blue doesn't mean all blue berries are blueberries".

In the example that you write, both the snake and Douglas Barr share only one commonality. Having one thing in common doesn't make them the same in total. Each belongs to the subset of "Animals without legs", of which many other animals would belong as well, but not to the specific set of "Snakes".

Answer 5

Well Douglas Bader was known to have a bad temper so maybe he was a rattlesnake?

Answer 6

Looks right to me.

Answer 7

Read the STORY of SIR Douglas Bader, - and, DO NOT insult the man's MEMORY, - PLEASE!

Answer 8

I think you got it right there.................WHAT?