 A major difficulty is whether one and the same science—metaphysics—can have such two distinct objects as God and creatures, still respecting the principle of the unity of science (DOS 221; see also QLIS 4). In DOS 226 Kilwardby claims that unity by analogy is sufficient for the science of being qua being (scientia de ente), metaphysics, to be the field of inquiry that has God and the creatures as objects of inquiry. Being and substance can be predicated by analogy of God and creatures: of God primarily and properly, of creatures in a participative and less proper way. God cannot be known directly, but He can be known by means of His effects and by negation, in the same way our bodily eyes do not see themselves except through a mirror (DOS 225).