 The most notable rebuttal that Heraclitus gave to such arguments was his famous quote: “No man ever steps in the same river twice.” (Patrick) This analogy is perhaps the clearest way to articulate his idea.
	Plato was not alone in his criticism of fire and flux. Parmenides, another prominent Greek philosopher, was emphatic about the principle of non-contradiction, a concept that simply states that contradictory statements cannot be simultaneously true. (Cohen) He concluded that Heraclitus was wrong, and objected that change leads to unknowability. He suggested an alternative theory regarding the composition of all things, sometimes referred to as “the One.”
	This concept rejects the notion of plurality of objects. Parmenides believed that the One of which all else was comprised is eternal, changeless, and homogenous. According to his writings, our senses deceive us, and we must only perceive change in objects, because an actual change in composition cannot exist.
As you can see, the Atomists did not walk into a quiet room of people waiting to hear their revolutionary (or at the time, farfetched-sounding) idea. So if Democritus’ ideas about atoms were to hold up against the arguments of Heraclitus and Parmenides, he had some explaining to do. Let us look then at the primary problems that the aforementioned concepts would present for the Atomists, as well an Atomist response to them.
Heraclitus suggested that everything is made of fire, and is constantly changing. On one point, Democritus would agree. The Atomist hypothesis included the idea that atoms are constantly moving, which accounts for the flux portion of the fire and flux theory. There is, however, no agreement between the two parties regarding fire. Heraclitus’ concept of fire being the root of all things begs the question, from what is fire made? An Atomist would respond, of course, that all things, including fire, are comprised of the same basic structures. And while you may not be able to step into the same river twice, your foot will still in fact be wet.
On the unchangeable structure of atoms, Parmenides might agree with the Atomists (despite an obvious discrepancy in semantics). The biggest dispute between Parmenides and the Atomists would be the notion of void. Parmenides rejected the idea of movement on the premise that for there to be movement there necessarily must be a void, but a void is nothing, and a nothing cannot exist because of the principle of non-contradiction. Historically, the Atomists mostly disregarded this argument, based on the simple reality that movement can be observed. (Russell) But in theory, one could certainly argue in defense of the atomistic concept that a void is not necessarily nothing, and that it is instead a material (perhaps too comprised of atoms) that is between the visible, and that just like wind, the inability to perceive it does not inherently negate its existence.
There are certainly some valid ideas represented in the theories of both Heraclitus and Parmenides. 
