6 “Indignation to them after the similitude of a serpent”. A great thing you are to hear. “Indignation to them after the similitude of a serpent.” As if we had said, What is that which you have said? There follows, “As if of a deaf asp.” Whence deaf? “And closing its ears.” Therefore deaf, because it closes its ears. “And closing its ears.” “Which will not hearken to the voice of men charming, and of the medicine medicated by the wise man”. As we have heard, because even men speak who have learned it with such research as they were able, but nevertheless it is a thing which the Spirit of God knows much better than any men.
For it is not to no purpose that of this he has spoken, but because it may chance that true is even that which we have heard of the asp. When the asp begins to be affected by the Marsian charmer, who calls it forth with certain peculiar incantations, hear what it does....Give heed what is spoken to you for a simile's sake, what is noted you for avoidance. So therefore here also there has been given a certain simile derived from the Marsian, who makes incantation to bring forth the asp from the dark cavern; surely into light he would bring it: but it loving its darkness, wherein coiled up it hides itself, when it will not choose to come forth, nevertheless refusing to hear those words whereby it feels itself to be constrained, is said to press one ear against the ground, and with its tail to stop up the other, and therefore as much as possible escaping those words, it comes not forth to the charmer.
To this as being like, the Spirit of God has spoken of certain persons hearing not the Word of God, and not only not doing, but altogether, that they may not do it, refusing to hear.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)