  In the Twenty Verses, Vasubandhu offers an elaborate defense of idealism. His arguments may be summarized as follows: Because non-existing entities can be consciously apprehended, consciousness must have ontological priority (VVS I, 1). The typical realist objection to an assertion of this sort concerns the causal function of perception: indeed, if mental content can be said to consciously arise without any reference to external objects, then why does it arise at a specific time and place and not everywhere and at any given moment? (VVS ad 2) In his response, Vasubandhu appeals to the dream argument: objects in a dream appear as having determined spatio-temporal coordinates without there being any correspondence to external realities. As he explains, dream objects arise in dependence upon the transformation of residual impressions (vāsanās), stock examples of which include the phenomenon of nocturnal emission of semen (VVS ad 3–4). Similarly, to the objection that cognition  must have a basis (āyatana) for its arising, Vasubandhu responds by appealing to the theory of seeds in the mental stream:  the physical object and the cognitive faculty which together provide  the basis for the arising of a particular type of cognitive awareness are now seen as an object-oriented cognitive aspect and its internal  representation. It only seems as though a patch of color and the  visual system which processes it are the basis for color qualia. In  reality, claims Vasubandhu, it is the residual traces (viz., the  transforming seeds) in the mental stream that take the form of  tangible object and tactile sense (VVS ad 9).