How does one define a genus? By it's genome? I would call an entirely artificial fruit 'an apple' if it were sufficiently similar to one.
I would call a raven a writing desk if it were sufficiently similar to one :P
But yeah, since we're really talking about human pattern-matching, apple is defined in terms of "common but not strictly necessary traits." Similar violations of any non-fuzzy definition could be constructed for shoes (hand-shoes), wire (a piece of wire shorter than it was wide), green (yellow), politicians (candidates), and hope (simple expression of preference). You might even be able to convince people that if you make a shape out of wood it can be a square even if the sides aren't exactly the same length. However, it is a convenient convention to omit this fuzziness in definitions since it's so common, and instead rely on the judgement of the reader to associate a thing with the closest definition.
Or, if there is no definition that is not different from the observed pattern in some key way ("key" here is subjective and mostly functional - a biologist might find genes of a species key but a non-biologist might find appearance key, and not vice versa), humans might make up a new category for this pattern.
I picked apple because it was the farthest from a true definition. For the others, I think one could use them in an 'if-and-only-if' manner and not be objected to.
hand-shoes
a piece of wire shorter than it was wide
I really want to see both of these things now.
Exercise for “Extensions and Intensions”
Give an intensional definition for each of the following words:
Now rank them from easiest to define to hardest.
Describe how you would give an extensional definition of the same words:
Again, rank them from easiest to hardest.
Are the two lists the same? If not, what tends to make something easier to define intensionally than extensionally and vice versa?
You can share your answers in the comments. I'm interested in seeing how similarly people think of these things. Please make suggestions as to how this could be improved or augmented and what to do the same/differently in future exercises. My current plan is to do more from the sequence "A Human's Guide to Words." This post will be edited in response to suggestions.