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Can you think of five famous people who you feel are rational/logical?. Deciding whether a syllogism is valid. Trying to decide if a syllogism is valid is not easy. Venn diagrams can help (at last a good use for Venn diagrams!). Deciding whether a syllogism is valid. Mmmmmmm…Vodka!. - PowerPoint PPT Presentation
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Can you think of five famous people who you feel are rational/logical?
Deciding whether a syllogism is valid
Trying to decide if a syllogism is valid is not easy.
Venn diagrams can help (at last a good use for Venn diagrams!)
Deciding whether a syllogism is valid
• Some IB students are from Russia
• All Russians are good at drinking vodka
• Therefore some IB students are good at drinking vodka
Is this a valid argument?
Mmmmmmm…Vodka!
Using Venn diagrams
• Some IB students are from Russia
IB Students
Russians
IB Students who are Russian
Using Venn diagrams
• All Russians are good at drinking vodka
IB Students
Russians
Good vodka drinkers
Using Venn diagrams
• Therefore some IB students are good at drinking vodka
IB Students
Russians
Good vodka drinkers
Another example
• All As are Bs
• All Bs are Cs
• Therefore all Cs are As
Another example
• All As are Bs
A
B
Another example
• All Bs are Cs
A
B
C
Another example
• Therefore all Cs are As
A
B
C
These Cs are not As
The syllogism is not valid
Another example!
• All As are Bs
• Some As are Cs
• Therefore some Bs are Cs
Another example!
• All As are Bs
A
B
Another example!
• Some As are Cs
A
B
C
Another example!
• Therefore some Bs are Cs
A
B
C
The syllogism is valid
Now your turn
Using Venn diagrams in your notebooks, decide whether each of the following arguments is valid or invalid.
Valid or invalid
• All Norwegians eat hotdogs
• Edward eats hotdogs• Therefore Edward is
Norwegian
Valid or invalid
• No Swedes have red noses
• Mr Luitjens has a red nose
• Therefore Mr Luitjens is not a Swede
Valid or invalid
• All IB1 boys are brave• Some brave people
are compassionate• Some IB1 boys are
compassionate
Valid or invalid
• Some physicists are frauds• Some frauds are not wealthy• Therefore some physicists are not wealthy
Valid or invalid
• All Norwegians have dogs• No good football players have dogs• Therefore no Norwegians are good football
players
Valid or invalid
• All clever girls are red-headed
• All red heads are rich• Therefore all clever
girls are rich
Valid or invalid
• No teachers are clever
• No IB1 students are clever
• Therefore no IB1 students are teachers