Prior knowledge or beliefs can influence reasoning if people neglect the form of the argument and concentrate on the content; this is referred to as the belief-bias effect. If a valid conclusion appears unbelievable, people reject it, while a conclusion that is invalid but appears believable is accepted as valid. Many people accept this syllogism as valid:
All oak trees have acorns. This tree has acorns. This tree is an oak tree.
Consider, however, this logically equivalent syllogism:
All oak trees have leaves. This tree has leaves. This tree is an oak tree.
In the first syllogism, people's knowledge that only oak trees have acorns leads them to accept the conclusion as valid. In the second syllogism, people's knowledge that many types of trees have leaves leads them to reject the conclusion as invalid.
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