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Digital thinking tools for better decision making
Digital thinking tools for better decision making

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1 Lewis Carroll, the master puzzler

Lewis Carroll (1832–1898), the British author of Alice in Wonderland and Alice through the Looking Glass, also published many logic puzzles. Here is one of them.

Suppose you are given the following starting points (called premises).

  1. All babies are illogical.
  2. Illogical persons are despised.
  3. Nobody is despised who can manage a crocodile.

What deduction can you make that uses all three of these premises?

One way to tackle this is by using Euler diagrams. The name comes from the Swiss mathematician Leonhard Euler (1707–1783), who had used them earlier than Venn.

The idea is that a set of things are represented by an oval. For example, this could be the set of babies. At this point you aren’t concerned with individual babies, just with babies as a class.

Described image
Figure 3 An oval labelled ‘babies’

You can also consider the set of illogical persons. Premise 1, All babies are illogical, means the set of babies must be entirely contained in the set of illogical persons. You can represent the relationship like this.

Described image
Figure 4 An oval labelled ‘illogical’ encloses the oval labelled ‘babies’

From Premise 2, Illogical persons are despised, you can deduce that the set of illogical persons is entirely contained in the set of despised persons.

Described image
Figure 5 An oval labelled ‘despised’ encloses the oval labelled ‘illogical’ which in turn encloses the oval labelled ‘babies’

Finally, Premise 3, Nobody is despised who can manage a crocodile, implies that no crocodile managers are despised. This is represented in the Euler diagram by showing the set of crocodile managers as having no overlap with the set of despised people.

Described image
Figure 6 Premise 3

From this, you can see that the set of babies cannot have any overlap with the set of crocodile managers, and you reach the conclusion that

No babies can manage crocodiles.

You can try this using another logic puzzle in the following activity.

Activity 1 ‘No ducks waltz’

Timing: Allow about 10 minutes

Consider the second Lewis Carroll logic puzzle below. What conclusion can you reach using all three of the premises given?

You could draw Euler diagrams, using pen and paper, but you may be able to solve the puzzle in some other way. If so, that’s fine. To solve it by Euler diagrams, you only need to use the two ideas you saw in the first example: sets enclosing one another and non-overlapping sets.

  1. No ducks waltz.
  2. No officers ever decline to waltz.
  3. All of Sue’s poultry are ducks.


From Premise 1 you can draw this diagram.

Described image
Figure 7 Diagram from Premise 1

From Premise 2 you can see that all officers waltz.

Described image
Figure 8 Premise 2 shows that all officers waltz

Premise 3 says that Sue’s poultry are a subset of ducks

Described image
Figure 9 None of Sue’s poultry are officers

So, there is no overlap between Sue’s poultry and officers, and you reach the conclusion that none of Sue’s poultry are officers.

In the next section you will learn about some operations you can do with sets.