Art historian Douglas Fordham has written a fascinating and very thoroughly researched article about Allan Ramsay's portraits of David Hume and Jean-Jacques Rousseau painted in 1766:
'Allan Ramsay's Enlightenment : or Hume and the Patronizing Portrait' Art Bulletin, September 2006.
These are undoubtedly two of the finest paintings of Enlightenment thinkers (and possibly the best painted portraits we have of any philosophers), yet there has been surprisingly little written about them. Fordham sets the portraits in their social and political context (including raising questions about imperialism) and provides an antidote to Edgar Wind's writing about Hume and the Heroic Portrait which seemed to miss quite a lot about the portraits and their original context. My speculative article about these two paintings 'Art and Allusion' even makes it into Fordham's footnotes, which is gratifying, given that it was published in The Philosopher's Magazine, and wouldn't be on the radar of most art historians - I put that down to the power of the Internet. For more on the context of these two paintings, see David Edmonds and John Eidinow's recent book Rousseau's Dog.