1....“Bless the Lord, O my soul! And all that is within me, His holy Name”. I suppose that he speaks not of what is within the body; I do not suppose him to mean this, that our lungs and liver, and so forth, are to burst forth into the voice of blessing of the Lord. There are lungs in our breast indeed, like a kind of bellows, which send forth successive breathings, which breathing forth of the air inhaled is pressed out into voice and sound, when the words are articulated; nor can any utterance sound forth from our mouth, but what the pressed lungs have given vent to; but this is not the meaning here; all this relates to the ears of men. God has ears: the heart also has a voice. A man speaks to the things within him, that they may bless God, and says unto them, “all that is within me bless His holy Name!” Do you ask the meaning of what is within you? Your soul itself. In saying then, “all that is within me, bless His holy Name,” it only repeats the above, “Bless the Lord, O my soul:” for the word “Bless,” is understood. Cry out with your voice, if there be a man to hear; hush your voice, when there is no man to hear you; there is never wanting one to hear all that is within you. Blessing therefore has already been uttered from our mouth, when we were chanting these very words. We sung as much as sufficed for the time, and were then silent: ought our hearts within us to be silent to the blessing of the Lord? Let the sound of our voices bless Him at intervals, alternately, let the voice of our hearts be perpetual. When you come to church to recite a hymn, your voice sounds forth the praises of God: you have sung as far as you could; you have left the church; let your soul sound the praises of God. You are engaged in your daily work: let your soul praise God. You are taking food; see what the Apostle says: “Whether you eat or drink, do all to the glory of God.” I venture to say; when you sleep, let your soul praise the Lord. Let not thoughts of crime arouse you, let not the contrivances of thieving arouse you, let not arranged plans of corrupt dealing arouse you. Your innocence even when you are sleeping is the voice of your soul.
2. “Bless the Lord, O my soul, and forget not all His rewards”. But the rewards of the Lord cannot be before your eyes unless your sins are before your eyes. Let not delight in past sin be before your eyes, but let the condemnation of sin be before your eyes: condemnation from you, forgiveness from God. For thus God rewards you, so that you may say, “How shall I reward the Lord for all His rewards unto me?” This it was that the martyrs considering (whose memory we are this day celebrating), and all the saints who have despised this life, and as you have heard in the Epistle of St. John, laid down their lives for the brethren, which is the perfection of love, even as our Lord says: “Greater love has no man than this, that a man lay down his life for his friends:” this the holy martyrs, then, considering, despised their lives here, that they might find them there, following our Lord's words when He said, “He that loves his life, shall lose it; and he that loses his life for My sake, shall keep it unto life eternal.”...“Forget not,” he says, “all His rewards:” not awards, but “rewards.” For something else was due, and what was not due has been paid. Whence also these words: “What,” he asks, “shall I reward the Lord for all His rewards unto me?” You have rewarded good with evil; He rewards evil with good. How have you, O man, rewarded your God with evil for good? You who has once been a blasphemer, and a persecutor, and injurious, hast rewarded blasphemies. For what good things? First, because you are: but a stone also is. Next, because you live, but a brute also lives. What reward will you give the Lord, for His having created you above all the cattle; and above all the fowls of the air, in His image and likeness? Seek not how to reward Him: give back unto Him His own image: He requires no more; He demands His own coin....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)