We have the following premises and associated Venn Diagrams:
"All dogs are mammals."
"All cats are mammals."
Combining the Venn Diagrams for premises 1 and 2, we get
the combined Venn Diagram.
However, if the deduction "all cats are dogs" is valid, then
region 9 should be
shaded.
But that is not the case for
the combined Venn Diagram.
So we can conclude that
the deduction "all cats are dogs" is not valid..
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