It was a commonplace for natural philosophers, the earliest scientists, to give lectures on the nature of things, proving that things exist, that we can have knowledge of them, and that we can communicate that knowledge through teaching.

Gorgias and Protagoras were critics of the Eleatic philosophers, especially Parmenides, who was Plato's inspiration for the idea that Truth must be eternal. Parmenides argued that Being must be unchanging. Being cannot be generated from nothing, nor can it cease to be.

Heraclitus disagreed, saying that everything was constantly changing ("All is flux" and "you cannot step in the same river twice"), but he held there was a "logos" governing change. In that respect, he anticipated the laws of nature.

The Eleatics were pre-cursors of the modern rationalists (Descartes, Spinoza, and Leibniz). Thomas Aquinas was a scholastic intermediary. They believed everything about the world could be deduced from first principles, working in an "ivory tower."

They were especially critical of the nature philosophers (physiologoi), who thought that knowledge could be gained by observing the material world. Perceptions of phenomena are unreliable, they said. Such knowledge is merely conventional, relative to each observer, not certain. The goal of philosophy is certain knowledge and truth.

The early materialist/physicalist philosophers Democritus and his teacher Leucippus replaced theological and supernatural explanations of phenomena with natural materialist explanations. They assumed the world was completely made of matter, which they postulated to consist of just a few types of invisible particles that could be combined to make all of the visible objects, their properties, and their behaviors.

The fundamental elements of their time - earth, water, air, and fire - were in turn simply compounds of sub-elementary particles they called atoms (indivisibles) in a void or vacuum between the atoms.

"By convention hot, by convention cold, but in reality atoms and void."
(Fragment 117, Diogenes Laertius IX, 72)

Parmenides had denied the possibility of the void with the simple logical/language argument that if "nothing" was between two bodies, it follows that they must be in contact with one another. Plato and Aristotle generally preferred Parmenides' idea of a continuous filled plenum and opposed the atomists' ideas of discrete particulate objects separated by nothing.

Democritus denied the arbitrariness of phenomena that was implied if they were the free actions of the gods. He replaced that explanation with the idea of deterministic laws governing the behavior of the atoms, and as a consequence explaining all phenomena made of atoms, including human beings and their actions. Leucippus had denied that anything happened at random (μάτην),

"Nothing occurs at random, but everything for a reason and by necessity."
(Leucippus, Fragment 569 - from Fr. 2 Actius I, 25, 4)

Gorgias' colleague Protagoras accepted the criticism of phenomena as merely conventional. The same wind might seem warm to one man and cool to another. But their perceptions of the phenomena are equally valid. "Man is the measure of all things," he said, where things (χρημα) refers to physical phenomena.

In his Theaetetus, Plato tells us that Socrates considered, but ultimately rejected, three possibilities for what knowledge (ἐπιστήμη) is and how we come to have it.

The first is perception (αἴσθησις). Our perceptions are "true" (ἀληθῆ), at least to us, a kind of private knowledge. But they may be dreams or illusions. (160D)

The second is true (ἀληθῆ) opinion or belief (δόξαν). Socrates asserts that Protagoras' relativistic argument that "man is the measure of all things," means "what is true is what is true for me." But "myriad" others may properly judge your opinion false (ψευδῆ).(170D)

The third is true belief that had some reasons (λόγος) or justification (συλλογισμῶ), a rational explanation for the belief. True (or right) opinion accompanied by reason is knowledge. (δόξαν ἀληθῆ μετὰ λόγου ἐπιστήμην εἶναι) (202C)

This third possibility that knowledge is "justified true belief" has come down to modern times as the three-part "traditional" theory of knowledge.

Although Socrates' "negative" dialectic never established any certain knowledge, Plato believed that Socrates' method of inquiry (ἔλεγχος) was the way to achieve knowledge.

Nevertheless, the Theaetetus ends with Socrates' utter rejection of perception, true belief, or true belief combined with reasons or explanations as justification. Socrates says:

And it is utterly silly, when we are looking for a definition of knowledge, to say that it is right opinion with knowledge, whether of difference or of anything else whatsoever. So neither perception, Theaetetus, nor true opinion, nor reason or explanation combined with true opinion could be knowledge (epistéme).

Gorgias was in some sense a skeptic, an early post-modern thinker, even a deconstructionist as to the power of language. He expressly used the language, the rhetorical style, and the arguments of the Eleatics to refute their claims about Being (το ον) and their denial of the existence of Non-Being (το μη ον)

He and Protagoras can be seen as attacking the view that thought alone can establish true knowledge. One must look at the world of material things. In this respect they are precursors of the modern empiricists (Locke, Berkeley, and Hume). Their medieval predecessor was John Duns Scotus, who insisted that the freedom of God meant that one had to look at the world to understand it. Despite the ancient Stoic dicta that God is Nature and that God is Reason, one can not understand the world with reason alone.

Many ancient physicists (φυσικοι, φυσιολογοι) lectured and wrote on "what there is" in treatises called "Peri Physis" (Περι Φύσις) - roughly, About Nature, or The Nature of the Physical World.

The content of the typical physicist/philosopher lectures was usually in three parts:

• Things exist
• You can know what things exist
• You can tell others about what exists

Gorgias is reported to have dazzled and delighted his audiences by proving the opposites, by using nearly identical arguments:

• Nothing exists
• If by chance something did exist, you could not know anything about it
• If you did accidentally learn something about it, there is no way you could communicate your knowledge to others

Note that the Gorgias claim "nothing exists" is not so extreme when it is seen as an attack on the Platonic/Parmenidean view that only Being really exists. Parmenides denies the existence of the void as Non-Being. Gorgias thus can be seen as providing a logical defense for the ephemeral phenomena that exhibit "Becoming" - coming and going - if all he claims is that "Non-Being" exists.

One (negative) lesson we can take away from Gorgias is that arguments, especially verbal reasoning alone, can be used to prove anything by clever rhetoricians. Logical and linguistic arguments can tell us nothing "true" about the physical world.

This is the problem of knowledge. How can we know - how can we be certain about - what we know? It is related closely to the question of how abstract concepts/thoughts and concrete physical objects (epistemology, ontology, cosmology) exist in the universe - how "ideal" thoughts relate to the material "things themselves." How is what we perceive through our senses related to the physical things and the abstract concepts that our reason tells us lie behind the laws of nature (metaphysics).

But the other lesson from Gorgias may simply be that the phenomena, changing as they do, including the atoms and void of the materialists, are a source of equally valid knowledge.