Why is it so easy to define "island" and so difficult to define "dog"? Both

Why is it so easy to define "island" and so difficult to define "dog"? Both

Why is it so easy to define "island" and so difficult to define "dog"? Both terms refer to quite "natural" and well-known things. We can say that an island is an area of land surrounded by water, but we can't say, for instance, that a dog is an animal that barks, since a sick dog that can't bark is still a dog. It is also curious that we all know what a dog is without knowing the zoological definition of this species. Is there a name for this difference between the two words?

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