Man with a Blue Scarf On Sitting for a Portrait by Lucian Freud by Martin Gayford Thames & Hudson, 256 pp., $40 Art critics have been sitting for their portraits since Diderot, grandaddy of modern criticism, modeled for Fragonard. Under 18th-century Prussian rigor, aesthetics hardened into a discipline. Critics arose as arbiters and exegetes. The benefits of painting them rose, too. Johann Winckelmann, pioneer of art historical methodology, posed for Anton Mengs; Immanuel Kant, for lesser lights. John Ruskin held his stance for John Millais. Continue Reading