To distinguish clearly, after the lapse of nearly five centuries, between Uccello and Castagno, and to determine the precise share each had in the formation of the Florentine school, is already a task fraught with difficulties. The scantiness of his remaining works makes it more than difficult, makes it almost impossible, to come to accurate conclusions regarding the character and influence of their somewhat younger contemporary, Domenico Veneziano. That he was an innovator in technique, in affairs of vehicle and medium, we know from Vasari; but as such innovations, indispensable though they may become to painting as a craft, are in themselves questions of theoretic and applied chemistry, and not of art, they do not here concern us. His artistic achievements seem to have consisted in giving to the figure movement and expression, and to the face individuality. In his existing works we find no trace of sacrifice made to dexterity and naturalism, although it is clear that he must have been master of whatever science and whatever craft were prevalent in his day. Otherwise he would not have been able to render a figure like the St. Francis in his Uffizi altar-piece, where tactile values and movement expressive of character—what we usually call individual gait—were perhaps for the first time combined; or to attain to such triumphs as his St. John and St. Francis, at Santa Croce, whose entire figures express as much fervour as their eloquent faces. As to his sense for the significant in the individual, in other words, his power as a portrait-painter, we have in the Pitti one or two heads to witness, perhaps, the first great achievements in this kind of the Renaissance.