„Whence has water so great an efficacy, as in touching the body to cleanse the soul?“ This famous question from Augustine's , Tractates on the Gospel of John‘ – rhetorically asked with regards to Baptism – was for centuries the object of concern for medieval theologians trying to explain the efficacy of the sacramental administration of grace. In particular, the striving for rationality within late medieval scholasticism seemed to encounter insurmountable limits, for, how should it be that a corporeal element such as water operated with something as spiritual as grace? How should a natural event such as the contact with water produce a supernatural purification of the soul? Augustine's own answer, that this was due to the belief in the accompanying words, was only one of several models that late medieval scholasticism developed and transferred from the question of sacramental efficacy to related problems such as the efficiency of holy water. Starting with a late scholastic tract on holy water, the , De efficacia aquae benedictae‘ of Juan de Torquemada (written ca. 1437 at the council of Basel), this contribution explores the different models of explanation for the special power of water and illustrates the role that the particular properties of water assumed for the development of a general theology of the sacraments.