I have dwelt first upon the subject of this decoration, because it is a key to so many of the old paintings and to many modern ones as well. Their subjects represent abstract ideas personified, embodied in human form; the particular idea being shown by the emblems which accompany each figure. People had come to recognise that such and such an emblem indicated such and such an idea, and, whenever a painter wished to suggest that idea, he represented a figure with the familiar emblem.