The Duties of the Heart, by Rabbi Bachye, tr. by Edwin Collins, , at sacred-texts.com
Man is made up of body and soul; and both alike are given us by the beneficence of the Creator. The one is visible, and the other invisible. We are therefore bound to serve Him with a twofold service. That of the body and its members can be fulfilled by the visible activities of man; but the second is a hidden service, which is the fulfilment of the duties of the heart—to acknowledge the Unity of God in our hearts, to believe in Him, to love Him, resign our souls to Him, and make His name the unifying central thought of all our conduct.