The main doctrines of the Eleatics were evolved in opposition, on the one hand, to the physical theories of the early physical philosophers who explained all existence in terms of primary matter (see Ionian School), and, on the other hand, to the theory of Heraclitus that all existence may be summed up as perpetual change.
As against these theories the Eleatics maintained that the true explanation of things lies in the conception of a universal unity of being.
Most famous in connexion with this kind of poetry are Xenophanes and Parmenides, the Eleatics and Empedocles of Agrigentum.
Illingworth has said very concisely: " The physical speculations of the Ionians and Atomists rendered a God superfluous, and the metaphysical and logical reasoning of the Eleatics declared Him to be unknowable."
Among the pre-Socratic nature-philosophers of Greece, Heraclitus and the Eleatics are the chief representatives of this polemic. The diametrical opposition of the grounds on which the veracity of the senses is impugned by the two philosophies (see Heraclitus, Parmenides, Eleatic School) was in itself suggestive of sceptical reflection.
His arguments were drawn from the dialectic which the Eleatics had directed against the existence of the phenomenal world.
Between Leucippus and Democritus there is an interval of at least forty years; accordingly, while the beginnings of Atomism are closely connected with the doctrines of the Eleatics, the system as developed by Democritus is conditioned by the sophistical views of his time, especially those of Protagoras.
While Leucippus's notion of Being agreed generally with that of the Eleatics, he postulated its plurality (atoms) and motion, and the reality of not-Being (the void) in which his atoms moved.
In some cases these transitions may be unsatisfactory and forced; it is apparent that the linear development from " being " to the " idea " is got by transforming into a logical order the sequence that has roughly prevailed in philosophy from the Eleatics; cases might be quoted where the reasoning seems a play upon words; and it may often be doubted whether certain ideas do not involve extra-logical considerations.
A further step, then, was necessary, and it was taken at any rate by the Eleatics, when they opposed their thought to the thought of others, as the way of truth in contrast to the way of opinion.
In this they differed from the Eleatics and the Pythagoreans who thought in the abstract, and explained knowledge and existence in metaphysical terminology.
While agreeing with the Eleatics as to the eternal sameness of Being (nothing can arise out of nothing; nothing can be reduced to nothing), Democritus followed the physicists in denying its oneness and immobility.
Being is the Full i pes, plenum); not-Being is the Void (nEvov, vacuum), the infinite space in which moved the infinite number of atoms into which the single:Being of the Eleatics was broken up. These atoms are eternal and invisible; absolutely small, so small that their size cannot be diminished (hence the name IITOµos, "indivisible"); absolutely full and incompressible, they are without pores and entirely fill the space they occupy; homogeneous, differing only in figure (as A from N), arrangement (as AN from NA), position (as N is Z on its side), magnitude (and consequently in weight, although some authorities dispute this).