7 And whereas, not only in action, but also in the thoughts of the mind, men are moved to deeds of virtue, he afterwards adds, saying, 'My eyes prevent the dawn, that I might meditate on Your words. ' For it is meet that the spiritual meditations of those who are whole should precede their bodily actions. And does not our Saviour, when intending to teach this very thing begin with the thoughts of the mind? Saying, 'Whosoever looks on a woman to lust after her, has already committed adultery:' and, 'Whosoever shall be angry with his brother, is guilty of murder. ' For where there is no wrath, murder is prevented; and where lust is first removed, there can be no accusation of adultery. Hence meditation on the law is necessary, my beloved, and uninterrupted converse with virtue, 'that the saint may lack nothing, but be perfect to every good work.' For by these things is the promise of eternal life, as Paul wrote to Timothy, calling constant meditation exercise, and saying, 'Exercise yourself unto godliness; for bodily exercise profits little; but godliness is profitable for all things, since it has the promise of the present life, and of that which is eternal.'
Source: Letters (New Advent)