I noticed that as well. If this kind of grammar error is incredibly rare in the space of GPT outputs, it could indicate a forgery. (I can’t believe I just applied that word to a generated text.)
That grammatical error was generated by GPT-3. A possible explanation is that there is a randomness factor that sometimes selects a word other than the most statistically likely next word and perhaps in this case created a grammatical issue.
Notice that the given prompt has an error as well, "GPT-3 on Philosphers by GTP-3", missing an "o" from "philosophers". Seeing the prompt, it may have adjusted itself to be more prone to make errors.
Btw. I'm not a native English speaker - is this sentence correct : "Is a cat a animal?"
Shouldn't it be "Is a cat an animal?" ?