Democritus' ideas represent the first formal attempt to create a materialism. Everything - even the soul - is composed of atoms. Hence all motion or change is local motion - the motion of atoms.
The question then arises: in what or within what does this motion take place? The void - the atoms are and move in the void. With Parmenides - and, apparently, with the Greeks at large, the void was understood as non-being - and hence as not being, - as impossible.
Democritus takes a major step in transforming the concept of (absolute) non-being into (relative) non-being - into the concept of space, - e.g., relative to what is full, the atoms. The void has, as it were, a spatial being
Knowledge - things emit a kind of spectre or subtle image (eidolon) which is composed of finer atoms. These penetrate the sense organs and thus the mind receives a copy or replica of the thing.
The wise man - the sophos - imperturbable, senene, in control over himself.
Kirk & Raven:
All things are composed of solid atoms, along with the space or void between them.
A-tom == in-divisible
They are incompressible and homogeneous. They differ in form, arrangement and size -- that is, only quantitatively, not qualitatively. The qualities that we distinguish in things are produced by the movement or rearrangement of these atoms.
Atoms are eternal and uncaused -- as is motion, which "Of its nature" must originate in preceding motion.
As everything is made up of these unchangeable and eternal atoms, it follows that coming into being and passing away are but a seeming, an appearance produced by the rearrangement of the atoms. [Consider the echo of Parmenides' rigid distinction between the Way of Being / Truth and the Way of Non-Being / Opinion.] The beings that you and I think we are, are but temporary aggregations of atoms, that will soon separate to enter into the substance of others beings of things. And yet, in ages of time, perhaps, we shall be re-formed, when it may so fall out that our atoms come together again. Thus history repeats herself endlessly.
It must be remembered that Democritus' atoms were in no sense the product of experimental investigation. His atoms, like their motion and like the void in which they moved, were alike hypotheses and based on no sort of exact knowledge or experience. The obvious parallels w/ the l9th ct. scientific doctrines concerning the "indestructability of matter" and the "conservation of energy" are more apparent than real.
Perhaps as the history of this school of thought shows, the philosopher/ scientists of this time were not primarily concerned with what we would recognize as science. The Epicureans (after Epicurus - 342-270 B.C.) showed little tendency to extend the range of scientific ideas.
W. T. Jones -
Jones' gives a good account here, I think, of how Empedocles and Anaxagoras represent failed efforts to provide an account of diversity in the world on the basis of assuming that there is a plurality of qualitatively diverse roots (e.g., a finite number [four elements] in Empedocles, or an infinite number [the homoiomeraiai] in Anaxagoras).
That these accounts were not satisfactory is clear; whether or not Jones is correct to assume that the pluralists were aware of the problem in terms of the failure of assuming a qualitative diversity is not so clear. In any case, he is correct to point out that atomism represents a shift to the more complicated, but perhaps - ultimately - more successful strategy of explaining things by way of simply (that's the point) quantitative diversity.
Infinite diversity - infinite numbers of infinitely small seeds - is also rejected - perhaps by way of Zeno who pointed out that if anything infinitely small "were added to any other thing it would not make it larger; for nothing can gain in magnitude by the addition of what has no magnitude." (Jones, p. 77)
Hence the concept of atoms - of very small particles of a given size and shape (hence not differentiated in terms of quality).
Moreover, the obvious difficulty is: how do the atomists get around Parmenides' argument against "nothing" - for "nothing" in some sense is necessary in order for the Parmenidean one to become the many atoms. Jones' suggestion seems to be correct - that the atomists establish a concept of empty space (the Void) - not as no-thing ["what -s-not, is not"], but as something.
This suggests a resolution to the Parmenidean argument for Being - that it rests on a fallacy of equivocation.