Logic issues

[pedestrian]

The case that I keep running into I would like to call 'proof by example'. It may have a proper name, but I don't know it.

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Suppose I hold a fruit behind my back and I ask someone to guess what it is, and I tell him that the fruit is yellow. The answer could be: it has to be a lemon, because a lemon is yellow.

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Or possibly just a syllogism:

The fruit is yellow, lemons are yellow, therefore the fruit is a lemon