Leucippus and Democritus
According to Democritus and Leucippus, the nature of reality consists in atoms and void.
On the existence of the ordinary objects salient in experience, Democritus and Leucippus may have had different views. The evidence for what Leucippus thought is less certain, but Democritus seems to have thought the ordinary objects salient in experience are how the atoms in the void appear to humans beings. Democritus seems to have thought that belief in the existence of these ordinary objects and the traditional conception of reality is a product of sense experience, that sense experience is a "bastard" form of judgment, that this form of judgment is not knowledge, that the ordinary objects salient in experience are the way arrangements of atoms in the void appear, and that only the "legitimate" judgment of reason provides knowledge of what exists.
This way of thinking about the ordinary objects in experience is striking. It is more typical to think that these objects come into existence, persist through various changes, and eventually go out of existence, but Democritus seems to have thought that this way of thinking mistakes appearance for reality. In reality, according to Democritus, there are no such objects. Democritus accepts Parmenides' conclusion that nothing comes into or goes out of existence. The atoms exist and move in the void, but the atoms do not come into or go out of existence. They become arranged in various ways as they move aimless through the void, but these transient arrangements are not themselves objects and hence are not objects that come into or go out of existence.
Democritus seems to have thought that the traditional conception of reality as consisting of objects that come into and go out of existence is a "custom" or "convention" that has its basis in the senses. It is a "bastard" judgment, not a "legitimate" judgment in terms of reason. Human beings typically rely on their senses for their beliefs, and it is through their reliance on senses that human beings form their belief in the traditional but mistaken conception of reality in which there are objects that come into and go out of existence. Democritus accepts Parmenides' injunction to "not let habit born from much experience (πολύπειρον) compel you along this way to direct your sightless eye and sounding ear and tongue, but [instead to] judge by reason (κρῖναι δὲ λόγῳ)."
"Democritus sometimes does away with what appears to the senses, and says that none of these appears according to the truth but only according to opinion. The truth in real things is that there are atoms and void. 'By custom,' he says, 'sweet, bitter, hot, cold, color; but in truth, atoms and void'" (Sextus Empiricus, Against the Mathematicians VII.135; DK 68 B 9).
"He says that there are two kinds of knowing, one through the senses and one through the intellect. Of these he calls the one through the intellect 'legitimate,' attesting its trustworthiness for the judgment of truth, and that through the senses he names 'bastard,' denying it inerrancy in the discrimination of what is true. To quote his actual words: 'Of knowing there are two forms, one legitimate, one bastard. To the bastard belong all this group: sight, hearing, smell, taste, touch. The other is legitimate and separate from that'" (Sextus Empiricus, Against the Mathematicians VII.138; DK 68 B 11).
(Sextus Empiricus is a Pyrrhonian Skeptic who lived 2nd or 3rd century AD. He is the primary source for Democritus' epistemology. Sextus discusses Democritus in Against the Mathematicians as part of a long review of theories of the criterion of truth. (A criterion of truth is a means for judging correctly that something is the case and thus for having knowledge.))
"The principles of the totality of things are atoms and the void, everything else is the object of convention. ... Nothing comes into being from what does not exist nor is it destroyed into what does not exist. And the atoms are unlimited in magnitude and number and they move in the whole, whirling around" (Diogenes Laertius, Lives of the Philosophers IX.44).
A Defense of the Inquiry into Nature
Democritus and Leucippus provide the means for a defense of the inquiry into nature against the criticism that Parmenides raises.
Thales and the Milesian inquirers into nature may follow a "backward-turning" method, just as Parmenides suggests. They may have thought that ordinary objects exist in terms of changes in the nature of reality, but Democritus seems to have thought this ontology is not essential to the inquiry into nature. Ordinary objects do not exist and so do not exist in terms of the nature of reality. The traditional conception of reality is how the nature of reality appears to human beings when they rely on sense experience rather than the "legitimate" judgment of reason. The inquiry into nature relies on reason to show how changes in the objects postulated in this traditional and customary way to conceive of reality are a matter of changes in the locations of atoms in the void.
The understanding of "reason" and the "habits of experience" in Parmenides and this defense of the inquiry into nature is not specified in much detail. Ordinarily human beings are said to exercise reason when they act sensibly as opposed to foolishly. The contrast between "reason" and the "habits of experience" in Parmenides is a contrast between the faculties of the mind involved in understanding how things really are as opposed to how they seem in ordinary life. More work is necessary to clarify and deepen the distinction between the cognition involved in "reason" and that involved in the "habits of experience" if this contrast is to be sustained. This project becomes a focus in the Period of Schools (the time of Socrates and Plato, Aristotle, and the Hellenistic philosophers).
Reason and Experience
Because so little of what Democritus thought about the soul has survived, it is unclear how he tried to clarify the contrast between "reason" and "experience."
"[H]ow does it come about that, if reality is the way Parmenides describes it, we nevertheless perceive it the way we do? It was Democritus who took up this problem, and, in taking it up, had to face the question of the relative roles of thought and perception in cognition. He thus, instead of having a vague and indefinite notion of some cognitive power of thought, came nearer to having a notion of reason by trying to determine more precisely the relative role of thought in cognition. Unfortunately Democritus' thought is preserved highly selectively, and there is not much evidence concerning his views on the soul. But given that he thought of philosophy as providing therapy for the afflictions of the soul, it would seem that he, too, had a substantive notion of the soul integrating perception, thought, belief, and desire in some systematic way" (Michael Frede, "Introduction" in Rationality in Greek Thought, 21-22).
"Medicine, according to Democritus, cures the illnesses of the body, but wisdom (σοφίη) removes the soul from its affections (παθῶν)" (Clement of Alexandria, Pedagogue 184.108.40.206).
(Aristotle thought that some of his Presocratic predecessors wrongly collapsed the distinction between sensation and intellect, and he cites Democritus as an example. Aristotle attributes to Democritus the view that "that soul and intellect are the same thing (ταὐτὸν ψυχὴν καὶ νοῦν)..." (On the Soul I.2.404a; cf. On the Soul I.2.405a). If Democritus held this view, it is not easy to see how he distinguished between the the "legitimate" and "bastard" forms of judgment. These forms of judgment seem to presuppose a distinction between sensation and intellect.)
Perseus Digital Library:
Henry George Liddell, Robert Scott, A Greek-English Lexicon:
ἄτομος, atomos, adjective, "uncut"
νόος, noos, noun, "intellect"
Arizona State University Library.
Loeb Classical Library: Early Greek Philosophy, Volume VII: Later Ionian and Athenian Thinkers, Part 2