Identity is an important metaphysical topic. All the objects in the world seem to always be changing. Heraclitus, an ancient Greek philosopher, famously said: "You cannot step in the same river twice." The river is always flowing, so talking about the river as if it has remained the same river is impossible. Heraclitus concluded that identity was not real, only change was. Parmenides, another philosopher, thought the opposite: change is not real, and is an illusion. To him, everything has always been the same object. This is a view called "monism".