Philosophical presentism is the view that only present entities exist (or, equivalently, that everything is present). According to presentism, there are no past or future entities. In a sense, the past and the future do not exist for presentists—past events have happened (have existed) and future events will happen (will exist), but neither exist at all since they do not exist now. Presentism is a view about temporal ontology that contrasts with eternalism—the view that past, present and future entities exist (that is, the ontological thesis of the 'block universe')—and with no-futurism—the view that only past and present entities exist (that is, the ontological thesis of the 'growing block universe'). Augustine of Hippo proposed that the present is analogous to a knife edge placed exactly between the perceived past and the imaginary future and does not include the concept of time. Proponents claim this should be self-evident because, if the present is extended, it must have separate parts—but these must be simultaneous if they are truly a part of the present. According to early philosophers, time cannot be simultaneously past and present and hence not extended. Contrary to Saint Augustine, some philosophers propose that conscious experience is extended in time. For instance, William James said that time is "the short duration of which we are immediately and incessantly sensible". Other early presentist philosophers include the Indian Buddhist tradition. Fyodor Shcherbatskoy, a leading scholar of the modern era on Buddhist philosophy, has written extensively on Buddhist presentism: "Everything past is unreal, everything future is unreal, everything imagined, absent, mental... is unreal. Ultimately, real is only the present moment of physical efficiency [i.e., causation]." According to J. M. E. McTaggart's "The Unreality of Time", there are two ways of referring to events: the 'A Series' (or 'tensed time': yesterday, today, tomorrow) and the 'B Series' (or 'untensed time': Monday, Tuesday, Wednesday).