19 “You spoke sometime in vision unto Your sons, and said”. You spoke in your vision. You revealed this to Your Prophets. For this reason You spoke in vision, that is, in revelation: whence Prophets were called seers. They saw something within, which they were to speak without: and secretly they heard what they preached openly. Then “You spoke in vision unto Your sons, and said, I have laid help upon One that is mighty.” You understand Who is meant by mighty? “I have exalted One chosen out of the people.” And Who is meant by chosen? One who, you rejoice, is already exalted.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)