Logic issues

[Stephen Carlson]

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. It is related to 'disproof by counter-example'.  Both may be valid in some cases, but are not valid in many other cases.

If one wants to prove that something is possible, then giving an example is sufficient of course.
However, let me illustrate the problem with an easy example.

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. This is what I would call 'proof by example'. It is obviously not valid, because it could also be a banana. 
It is not sufficient to argue about the likelihood of the two cases, because there are even more yellow fruit, some which the guesser is not even familiar with.

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