The Holy Spirit is that large river by which the mystical Jerusalem is watered. It is equal to its Fount, that is, the Father and the Son, as is signified in holy Scripture. St. Ambrose himself thirsts for that water, and warns us that in order to preserve it within us, we must avoid the devil, lust, and heresy, since our vessels are frail, and that broken cisterns must be forsaken, that after the example of the Samaritan woman and of the patriarchs we may find the water of the Lord.