The problem of the apparent tension between Being and appearance, between truth and opinion -- along with that between oneness and multiplicity -- is a problem "permeating all the concrete theorizing in philosophy after Parmenides."
Being || truth || oneness
---------- || ------- || ------------
Appearance || opinion || multiplicity
Philosophy in the era from Heraclitus to Democritus moves within the scope of Parmenides' notion of the Entity (to on) -- and represents a progressive splintering of Parmenides' idea (his predicates are retained and the essence of the concept of the Entity is unaltered) in order to introduce multiplicity into the Entity and make motion possible.
In this, Heraclitus represents "An attempt to re-interpret motion by changing it fundamentally from a notion of transition across discrete moments [which leads directly to Zeno's paradoxes] into a notion of continuous change."
(But this splintering is not sufficient -- the concept of Entity does not allow multiplicity. What will be called for will be a change in the conception of oneness -- Aristotle's idea of the hen, the one. This will be a unity which further allows for difference and multiplicity -- by way of the device of analogical and pros hen or focal equivocation. [CE])
[This comment may be helpful in that Heraclitus, contemporary with Parmenides and Xenophanes, nonetheless moves within the Parmenidean framework.]
"Heraclitus the Obscure"
In contrast with Parmenides, Heraclitus affirms the change or motion of all things: panta hrei, everything flows.
"No one can bathe twice in the same river -- the river endures, but the water is no longer the same."
Reality "in itself" is changing and unstable. Hence the primordial substance is fire -- the least consistent substance, the one which most readily transforms itself.
"War is the father of all things" -- which Marias takes to mean that discord, difference, contrariety, is the origin of everything in the world.
"The world is an eternal fire which transforms itself."
(We might say here: the early physicists and Parmenides seem bent on explaining the oneness of things in the face of their diversity by reaching back to some single, underlying unity; the emphasis is on unity and order -- so that the diversity of things, including their "changingness" is problematic. By contrast, Heraclitus suggests that such diversity and changingness is in fact the primary feature of the universe. This is a radical affirmation of appearance -- or, a complete inversion of the Parmenidean metaphysics.)
And so, on the ancient epistemological principle that "like is known by like," -- the dry soul, the one resembling fire, is the best soul, best at acquiring knowledge of the world as it is. The soul, like mud, is inferior.
(There is here, despite Heraclitus' move away from human notions of justice, etc. still a reliance upon the Milesian assumption of an analogy between the human and the natural.)
The Parmenidean influence can be seen, however, in his assertion that all things are one, and that nous (mind) is common to all.
Relatedly, H. introduces a new concept, to sophon -- "the wise one," but, according to Marias, meaning neither "the wise man" nor "wisdom." It is one, it is at all times, and it is separated from all things. Hence its predicates are those of the Parmenidean Entity.
H. advises that we be guided by nous (mind), as what is common to all men. "Those who are awake have a common world, but each sleeping person returns to his individual world."
Marias' interpretation: the waking man, who follows what is common to all (nous), is the one who reaches the sophon, which is one and always. There is by contrast the world of sleep, the individual world of each of us -- of opinion. It is in this latter world that everything is change and becoming.
"Nature likes to obscure itself" -- the world obscures the sophon, which is what truly is, separated from everything. Perhaps better, the sophon is nature -- it generates the world of becoming as its own veiling or obscuring, as something different from itself. It is then necessary to unveil the sophon to arrive at truth, the unveiling. [Marias' influence by Heidegger is especially apparent here.]
Since man is in the world, he is subject to becoming, and possesses something in common with the sophon, i.e., nous, which is divine -- especially if he has a dry soul. He does not become the sophon, which would be to become a god -- but only a philosopher.