17.10 John is singing: Answer
Answer for "John is singing"
is not a well-formed concept. The reason is that "John" is not a type but a certain individual with that name. Therefore, he belongs in a referent. But here, he is in the syntactic place occupied by the type.
Recall that a concept is made up of a type and a referent, and they are separated by a colon:
Thus we need to specify a type for John, and put "John" in the referent after the colon. For example, the all of the following would do in this context:
[Person: John] [Man: John] [Singer: John] [Tenor: John]
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