 The ability to fulfill one’s moral duty depends on divine grace, however, and is an aspect of pietas related to the Catholic doctrine of Free Will. Thus human beings have a capacity for piety and a moral duty as well as the power to do good, although their power is limited and dependent on the efficacy of divine grace. Erasmus is emphatic about this aspect in his definition of the philosophia Christi, that is, the pursuit of pietas. In a letter to Jan Slechta (1519), he writes: