James George Frazer,
The Golden Bough
(1922)
“ If there is any truth in this conjecture, it may explain very simply the origin of the double head of Janus, which has so long exercised the ingenuity of mythologists. When it had become customary to guard the entrance of houses and towns by an image of Janus, it might well be deemed necessary to make the sentinel god look both ways, before and behind, at the same time, in order that nothing should escape his vigilant eye. For if the divine watchman always faced in one direction, it is easy to imagine what mischief might have been wrought with impunity behind his back. ”
