From Rabbi Avraham ben HaRambam (Maimonides), Sefer HaMaspik (“The Guide to Serving God”), translated by Rabbi Yaakov Wincelberg (Feldheim Books), pp. 491-493. Although footnotes have been omitted, we have added explanatory information in parentheses.
RETREAT is an ideal path to intimacy with God. It is the way of the greatest chasidim (men of devotion), and through it, the prophets achieved Encounter with God. There is outward retreat, and there is inward retreat. The purpose of outward retreat is to realize inward retreat, which is the highest rung in the ladder toward Encounter, and is [a degree of] Encounter itself.
Inward retreat is the complete focus of the heart. David prayed to realize it, saying, "Create for me, God, a pure heart, and renew a proper spirit within me" (Tehillim 51:12). Assaf realized it, as he said, "My flesh and my heart are emptied; God is the rock of my heart and my portion” (ibid. 73:26). [This requires one] to empty the heart and mind of all besides God and to fill and occupy them with Him.
This is accomplished by totally or partially quieting the sensitive soul; detaching the appetitive (i.e. desiring) soul from the rest of one's worldly occupations and reorienting it toward God; filling the rational soul with God; and [finally] using the imaginative soul to assist the intelligence in its contemplation of God's magnificent creations, which testify to their Creator: the majesty and awe of the sea, with its wondrous creatures, the rotation of the great celestial sphere, the nature of the stars, and such.
Assaf described a heart and mind empty of all but God: "My flesh and my heart are emptied." Concerning the detachment of one's desires from the mundane, he said, "Who [besides You] have I chosen in the Heavens? And besides You, I desire nothing on earth” (ibid. 73:25). Concerning the orientation of all desire toward God, Yeshayahu said, "Your Name and the remembrance of You are the desire of the soul. My soul desires You in the night, even my breath within me seeks You" (Yeshayahu 26:8-9). And David said, "My being thirsts for You, my flesh longs for You" (Tehillim 63.2); "My being clings to You..” (v. 9). Alluding to the quieting of the senses, it is said," I have made a covenant with my eyes.. . ' (Iyov 31:1); "He shuts his eyes from seeing evil" (Yeshayahu 33:15). And David said, "Avert my eyes from a futile sight" (Tehillim 119:37). Concerning the need to restrain the senses and desires in order to focus the energy of the mind, Elisha said to Gechazi, "When you meet a man, do not bless him; when a man blesses you, do not respond" (Melachim II, 4-29). Understand this well.
The prophets and their students used instrumental music an song to achieve inward retreat, awaken desire toward God, and purify their interior from all besides Him. It similarly says of the order of the Temple service -- may it soon be restored! -- "And David and his generals designated for service the sons of Assaf, Heiman, and Yedusun, who prophesied with harps, lyres, and cymbals" (Divrei HaYamim 125:1).