Final answer:
The mystical or affective literature of the fifteenth century associates a 'pale face' with beauty. This understanding can help in interpreting Du Fay's song in the Missa Se la face ay pale.
Step-by-step explanation:
The mystical or affective literature of the fifteenth century indicates that a 'pale face' was often associated with beauty, purity, and a sense of spirituality.
In this context, a pale face could symbolize the idealized image of a virtuous and angelic figure. Applying this understanding to Du Fay's song in the Missa Se la face ay pale, it suggests that the lyrics may be highlighting the ethereal qualities and celestial beauty of the subject.