 The first view, represented on the left, is the ontologically austere view called presentism, the view that only the present exists. The past has been but is no longer, while the future will come to be but is not yet. Note that it is the convention of these diagrams that one spatial dimension is suppressed. The present is actually a three dimensional global slice of the spacetime. Moreover, the illustration necessarily represents the spatial extent of the present as finite and may suggest that time also has a beginning and/or end. These views are, however, merely artifacts of the representation and not integral to presentism, possibilism, or eternalism. The diagram illustrating presentism also has four arrows pointing towards the top of the page (conventionally taken to represent the future direction) attached to the plane representing the present. These arrows are meant to indicate something that is integral to presentism, the idea that the present (and hence the existent) constantly shifts or changes. These arrows represent, then, the dynamic aspect of time called temporal becoming or passage. It is widely thought that the deepest problems in the metaphysics of time concern the understanding of passage or temporal becoming and its relation to existence.