I take this to be the greatest poem in the Psalter and one of the greatest lyrics in the world. Most readers will remember its structure; six verses about Nature, five about the Law, and four of personal prayer. The actual words supply no logical connection between the first and second movements. In this way its technique resembles that of the most modern poetry. A modern poet would pass with similar abruptness from one theme to another and leave you to find out the connecting link for yourself. But then he would possibly be doing this quite deliberately; he might have, though he chose to conceal, a perfectly clear and conscious link in his own mind which he could express to you in logical prose if he wanted to. I doubt if the ancient poet was like that. I think he felt, effortlessly and without reflecting on it, so close a connection, indeed (for his imagination) such an identity, between his first theme and his second that he passed from the one to the other without realising that he had made any transition. First he thinks of the sky; how, day after day, the pageantry we see there shows us the splendour of its Creator. Then he thinks of the sun, the bridal joyousness of its rising, the unimaginable speed of its daily voyage from east to west. Finally, of its heat; not of course the mild heats of our climate but the cloudless, blinding, tyrannous rays hammering the hills, searching every cranny. The key phrase on which the whole poem depends is "there is nothing hid from the heat thereof". It pierces everywhere with its strong, clean ardour. Then at once, in verse 7 he is talking of something else, which hardly seems to him something else because it is so like the all-piercing, all-detecting sunshine. The Law is "undefiled ", the Law gives light, it is clean and everlasting, it is "sweet". No one can improve on this and nothing can more fully admit us to the old Jewish feeling about the Law; luminous, severe, disinfectant, exultant. One hardly needs to add that this poet is wholly free from self-righteousness and the last section is concerned with his "secret faults". As he has felt the sun, perhaps in the desert, searching him out in every nook of shade where he attempted to hide from it, so he feels the Law searching out all the hiding-places of his soul.
C.S. Lewis, Reflections on the Psalms, p. 62-64