LogFAQs > #933172719

Firstly, fish are not mammals.

That aside, it does not logically follow since there is no direct link between your A (all fish are mammals) and your C (some fish are poisonous). While it is certainly possible that some A are C, it's equally possible that ALL A are C or NO A are C. The subsets can exist within the larger set while being independent of each other.

For example, fish are mammals, some mammals breath fire, therefore some fish breath fire. This would not work if all mammals that breathed fire were bats. Likewise, if there was only two species of fish and neither of them breathed fire then the conclusion would be completely false. The only defining characteristic of a mammal is the presence of mammalry glands (hence why fish are not mammals). Since that does not bear a relationship to the conclusion the overlap between 'fish' and 'poisonous animals' may be of any amount ranging from 0 to 100% since they're not related.

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I am the gentle hand who heals, the happy smile who shields, and the foot that will kick your ***! - White Mage