Thanks for the question! The problem with the reasoning here is that we have two potential options before us: purely formal qualities (PFQ) or what the painting means (WPM). The author concludes that the aesthetic value of the painting lies in the second option, however, because there isn’t a compelling general account for PFQ. The author fails, however, to present any other compelling general account for WPM, instead concluding WPM solely based on the fact that there’s not a good argument for WPM. This, of course, doesn’t rule out PFQ, since there might be some argument for it that no one’s made it. Additionally, as I stated, it doesn’t entail that there’s a good positive argument for PFQ. This is the questionable reasoning present in this argument. Hope this helps! Feel free to ask any other questions that you might have.