S. Maximus: in the Same, p. 152.
Willing is a natural power, which desires what is in accordance with nature. Willing is a natural appetency, corresponding with the nature of the rational creature. Willing is a natural spontaneous movement of the self-determining mind, or the mind voluntarily moved about anything. Spontaneity is the mind moved naturally, or an intellectual self-determining movement of the soul.
Source: Fragments (New Advent)