Passage Summary: An art professor says we shouldn't listen to a critic's complaints about historical categories because the critic uses those same categories in his own music research.

Conclusion: The critic Costa's argument against the use of period styles in art history should be ignored.

Reasoning: Costa is inconsistent because his own work on French opera relies on the very same method of assigning works to historical periods that he claims is intellectually bankrupt.

Analysis: The professor commits a classic 'ad hominem' fallacy, specifically a 'tu quoque' or appeal to hypocrisy. Instead of addressing the logical merit of Costa's claim—that period styles are invalid because they lack universal defining features—the professor simply attacks Costa's personal consistency. In your search for the right answer, look for a description of how the professor attacks the person making the argument rather than the logic of the argument itself. It is a classic case of 'do as I say, not as I do' being used as a logical shield.