The Duties of the Heart, by Rabbi Bachye, tr. by Edwin Collins, [1909], at sacred-texts.com
(From the Gate of Unity.)
Belief in the Existence of One Creator as the Basis of Ethics
Perfect recognition of the existence and unity of God forms the only sure basis for right thought and conduct. For gratitude to the beneficent Creator of the universe, and admiration of creation, involve the reasoned resolve to benefit all His creatures, study His works, and raise our souls, perfecting them; and tends to love of Him, resulting in obedience to His will, whether revealed through Nature or through human agency, or in our innate faculties for recognising justice and the good.
The love of God, and the consequent acceptance of the yoke of duty in obedience to His will, is, in the Scripture passage that strikes the key-note of the faith of Israel, placed after the injunction to understand that the Creator exists, that the Eternal is God, and that He is One. (Hear and understand, * O Israel, the Eternal is our God, the Eternal is One) (Deut. vi.).
The unity of conduct, and the concentration of our love and striving after good, require belief in One, and only one, Creator, the source of good.
Footnotes
23:* Bachye is only following many Hebraists, and not at all straining the sense, when he renders "Shemang," in this verse, "Understand," or hearken, instead of "hear."