17 “I will render to You my vows, which my lips have distinguished”. What is the distinction in vows? This is the distinction, that yourself thou censure, Him thou praise: perceive yourself to be a creature, Him the Creator: yourself darkness, Him the Enlightener, to whom you should say, “You shall light my lamp, O Lord my God, You shall enlighten my darkness.” For whenever you shall have said, O soul, that from yourself you have light, you will not distinguish. If you will not distinguish, you will not render distinct vows. Render distinct vows, confess yourself changeable, Him unchangeable: confess yourself without Him to be nothing, but Himself without you to be perfect; yourself to need Him, but Him not to need you. Cry to Him, “I have said to the Lord, My God are You, for my good things You need not.” Now though God takes you to Him for a holocaust, He grows not, He is not increased, He is not richer, He becomes not better furnished: whatsoever He makes of you for your sake, is the better for you, not for Him that makes. If you distinguish these things, you render the vows to your God which your lips have distinguished.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)