Is taking ideas from an art critic plagiarism? A student submitted a paper in an art history class that contained some ideas from art critics she read about, and whose ideas she agreed with. The student didn't cite the critics as source, but claimed it wasn't plagiarism because their ideas were merely their own subjective judgments, or opinions, not facts or findings; furthermore, they were opinions she agreed with.
Yes, it's plagiarism if she learned novel or interesting ideas from an art critic and then used them in her own paper without crediting the source.  It doesn't matter whether the ideas are facts or opinions.

For commonplace opinions with nothing novel or interesting about them, there's no need for a citation.  However, that's presumably not the case here, since one couldn't attribute such opinions to a specific critic.