The early Greek philosophers generally believed that the universe (and therefore time itself) was infinite with no beginning and no end. In the 5th Century BCE, the Sophist philosopher Antiphon asserted that time is not a reality (hypostasis), but a concept (noêma) or a measure (metron). Also in the 5th Century BCE, Parmenides saw time (as well as motion and many other everyday things that we take for granted) as nothing more than an illusion because, he argued, all change is impossible and illusory (time as an illusion is also a common theme in Buddhist thought). Parmenides, then, believed that reality was limited to what exists in the here and now, and the past and future are unreal and imaginary. His near-contemporary Heraclitus, on the other hand, firmly believed that the flow of time is real and the very essence of reality.