 The second metaphysical issue that has a crucial bearing on time perception is connected with the A/B-theory dispute, and that is the debate between presentists and eternalists. Presentists hold that only the present exists (for an articulation of various kinds of presentism, and the challenges they face, see Bourne 2006), whereas eternalists grant equal reality to all times. the two debates, A- versus B-theory and presentism versus eternalism, do not map precisely onto each other. Arguably, B-theory is committed to eternalism, but A-theorists may not necessarily endorse presentism (though Bourne argues that they should).