The central meaning of emptiness in Yogācāra is a twofold "absence of duality." The first element of this is the unreality of any "conceptual duality" such as "physical" and "non-physical", "self" and "other". To define something conceptually is to divide the world into what it is and what it is not, but the world is a causal flux that does not accord with conceptual constructs. The second element of this is a "perceptual duality" between the sensorium and its objects, between what is "external" and "internal", between subject ("grāhaka," literally "grasper") and object ("grāhya," "grasped"). This is also an unreal superimposition, since there is really no such separation of inner and outer, but an interconnected causal stream of mentality which is falsely divided up.