And on His law he meditates day and night. Meditating is an exclusive trait of human beings, for even beasts appear to fancy and to think. Therefore the ability to meditate belongs to reason. There is a difference between meditating and thinking. To meditate is to think carefully, deeply, and diligently, and properly it means to muse in the heart. Hence to meditate is, as it were, to stir up in the inside, or to be moved in the innermost self. Therefore one who thinks inwardly and diligently asks, discusses, etc. Such a person meditates. But one does not meditate on the law of the Lord unless his delight was first fixed in it. For what we want and love, on that we reflect inwardly and diligently. But what we hate or despise we pass over lightly and do not desire deeply, diligently, or for long. Therefore let delight be first sent into the heart as the root, and then meditation will come of its own accord. It is for this reason that the ungodly do not meditate on the law of the Lord, since as false plants they did not take root. Yet they meditate on other things, namely, on things in which their delight is rooted, things they themselves desire and love, such as gold, honor, and flesh. But the Jews meditate on vanities and false frenzies according to their own ideas about the Scriptures, as has been prophesied concerning them in a variety of ways. But David prays (Ps. 119:36): “Incline my heart to Thy testimonies and not to gain!” All of the following do not meditate on the Law, but outside the Law: the greedy, the carnal, and the arrogant. Or they meditate on glosses of the Law, or dross and hulls.
But what is the meaning of day and night? Here is a large sea. First, literally, it means at all times, or incessantly, since day and night comprise all of time. However, time is divided not only into day and night but, second, also into good times and bad times. Thus the day is the time of prosperity, while night is the time of adversity. Third, it is divided into a time of grace and a time of sin. Fourth, into a time of life and of death. Prov. 31:18 reads: “Her lamp does not go out at night,” that is, in the time of death. Fifth, into a time of quiet and of activity, or rest and work. Hence quiet (according to the spirit) is the day, but activity is the night. According to these distinctions there is a variety of uses of these words in the Scriptures. Therefore he who is rooted willingly and spontaneously in the law of the Lord, no matter what time it is, does not go back, does not forget, does not put off meditation on the law of the Lord. The fool, however, and he whose delight is not in His law, changes at every difference in time. And though he may meditate on it sometimes by day, he stops at night, because he has no root (Luke 8:13).
On Psalm 1.
Luther’s works, vol. 10: First Lectures I