Presentation on theme: "Can you think of five famous people who you feel are rational/logical?"— Presentation transcript:
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? Mmmmmm m…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