Heraclitus believed in the unity of opposites. The succession of the opposites brings out his key notion of change. The successive manifestation of contrary properties in an object is a way of saying that everything undergoes change. All, things, according to Heraclitus, are in a constant flux. Comparing this change to stepping into a river, he says: you cannot step twice into the same river. I agree with this proposition considering the fact that the molecular property of a river at an instance is not the same, since it is constantly flowing. The constant flowing of the river suggests a constant movement of molecules so that new molecules interact with objects the river is in contact with. One may return to the same river, but fresh waters have flowed into it, making it different.
With this notion of change, it can be said that nothing retains its identity, though it remains the same thing. The object therefore endures, even though it is undergoing constant change and some of its components or characteristics may be lost. I think that the unity of the opposites therefore is a necessity for the existence of equilibrium, so that though in opposition, they maintain a balance and order in an object, and in nature as a whole. This unity which brings about balance and order is what Heraclitus calls the logos. Parmenides views are a contrast to those of Heraclitus; a sharp turn around the notion of change. He denies the reality of change, motion and void. For him, change is impossible and incoherent. All existence is permanent, ungenerated, indestructible and unchanging. In his view, there are no opposites, no plurality. For him, change and motion were mere illusions.
He favored pure reason as a path of understanding the world and its nature. He argued against the existence of void, equating it with non-being. For Parmenides, what is must exist, and what is not cannot exist, and is completely unlearnable. Only two things underlie reality for Parmenides: being and non-being. Anything that can exist and can be thought about must exist. It is therefore impossible to think or speak about what does not exist. Therefore, nothing cannot exist, and nothing can come into existence if it is not. For him, there is no difference between past, present and future. Therefore what is, already exists, and will exist, without change. What is, is therefore necessary. Parmenides names the logos of Heraclitus, being.
He opposes two possibilities for thought, being and non-being, and says that being is the only way that can be for thinking. The most non-being could do would be to not be. The appearances referred to as illusion or delusion are like what Heraclitus calls the flux and flow of reality beings coming to be and passing away; this permeates all reality; but in Parmenides view, they must only and can only return to being at what is perceived as destruction. For particular theorists, atomists in particular, the permanence of Parmenides and flux of Heraclitus are reconciled, and the atomic theory was conceived. I think this is their most important contribution to later metaphysicians.