Notes 5.2
Venn Diagrams are the easiest way for determining validity in categorical syllogisms.
All you have to do is put three circles together as follows:
Now, the procedure is simply to transfer the content of the premises to the circles and see
whether the premises guarantee the truth of the conclusion.
(Note: remember that
validity is where, if we assume true premises, the conclusion has to be true, too.)
If we
fill in the premises onto the three circles and the conclusion is impossible to fill in, then
we know that the corresponding syllogism is invalid.
Practice is generally the only way to pick up this technique, but there are a few rules we
can give to make things somewhat easier before you even start.
1.
Marks (that is, shading or making a big X) are entered only for the premises.
No
marks are necessary for the conclusion
2.
If the argument contains a universal premise, it must be entered first on the
diagram.
If there are two universal premises, then no order is required.
3.
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 Spring '08
 Barrett
 Logic, currently living tyrannosaurs

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