The Duties of the Heart, by Rabbi Bachye, tr. by Edwin Collins, , at sacred-texts.com
The word One, in common speech, denotes both accidental, transitory unity and a unity that is essential and permanent. It is this essential and permanent unity that is true unity . . . Even where the word one, in the sense of accidental unity, does not refer to obviously collective units or composite wholes, such as species, one army, etc., but is spoken of one man, even then it denotes a plurality.
For it denotes a being composed of substance and form, of what is essential and what is accidental; a being to whom existence and the loss of existence come from without; a being subject to composition and dissolution, change and subdivision, to classification, or association with others of like kind. Such unity is not true unity. Lay this well to heart.
Even when denoting absolute unity, the word One may be applied to a thing that is one of a number. . . .
But essential, absolute, real, objective unity, that is unchangeable and cannot be multiplied, and cannot be numbered among the qualities of matter, to which being or the loss of being cannot be caused from without, that is without beginning and without end, that cannot be moved and cannot be compared to anything or have anything compared with it, or be classed with anything, because it is One and unique from every point of view and in every aspect: this alone is the true unity, the root and cause of all collective or composite things. Such is the significance of the word One, when we speak of the unity of God.