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Examples of logic

This is a valid argument:

John is a human.
All humans have brains.
Therefore, John has a brain.

Student thinking

This is an invalid argument. The conclusion does not follow from the premises:

John is a fast runner. 
John is a University of Sussex student.
Therefore, all University of Sussex students are fast runners.

Sometimes, the conclusion does not follow from the premises but in isolation it is still valid. In this case though, the argument is invalid:

All seagulls are birds.
Some birds fly.
Therefore, all seagulls fly.

Next page - How not to construct arguments