Asked by Mike

All flies are winged creatures.
All butterflies are winged creatures
Therefore, all butterflies are flies

I believe this is an invalid syllogism, not 100% sure. Can you explain why it would be valid or invalid please
Answered by jim
A valid syllogism is:

All A are B
C is A
therefore
(since C is A and all A are B)
C is B

The non-syllogism above is:

All A are B
All C are B
Therefore all A are B.

Erm, no.

Spot the difference. Here's one of the same form:

All fish are animals
All birds are animals
Therefore all fish are birds.
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