Answer: a scientist and critic.
Step-by-step explanation: an appositive is a literary device that consists in a noun phrase or a noun that defines or explains another noun, which it follows, we can also say that it is two nouns or noun phrases that are together in a sentence and each one gives more information about the other. In the given sentence we have the noun "John" and the noun phrase "a scientist and critic" each one of them gives information about the other, so it is an example of an appositive.