The logical and sceptical bias of the Eleatics is surpassed by the hair-splitting dialectics of Zeno, whose evidences against motion and multiplicity still perplex the thinkers of to-day. On the one hand this precise manipulation of the laws of thought which represents the culminating point of Ionic rationalism redeems the negative Sophism which was beginning to deny the actuality and perceptibility of things themselves (Protagoras, Gorgias), while on the other hand the positive result of this strict definition of the highest conception of Being was to call forth a series of systems which came into existence almost simultaneously, though subject in part to reciprocal influence, a little before the middle of the fifth century. Such was the Doctrine of the Elements taught by Empedocles of Agrigentum, who once more found the idea of the imperishable principle in the fourfold root of Being (the four elements) and brought about the Heraclitic alternation of the external world by the introduction of the two polar forces of love and hate. The idea of the Element in endless subdivision (which could not be evaded in the world-process of Empedocles) and in endless diversity of quality was strongly brought out by Anaxagoras the Ionian in his _homoiomere_. To this chaos he opposed the thinking and directing reason (_nous_) as a distinct existence, thus definitely breaking with the idea of a hylozoistic union of matter and force, which had already threatened to go to pieces in the systems of Heraclitus and Parmenides, and setting forth the positive dualism of God and the world, _i.e._, of the Universal Reason working towards predetermined ends and the blind chaotic mass of matter. More important than either of these two is Leucippus of Miletus, the founder of the atomistic theory, who, as Theophrastus rightly asserts, starts from the position of Parmenides. For he finds the homogeneous, eternal, complete, and indivisible, unchangeable Existence, to which no quality can be ascribed, in the “atom,” and solves the difficulties which arose for the Eleatics out of the idea of multiplicity by assuming the existence of an infinite number of such units. Hence results a mechanical interpretation of nature, which proved of all ancient systems the most serviceable for the elucidation of physical and physiological facts. By explaining sensory impressions by mechanical transmission from object to subject, he propounds the first theory of sensory perception, and since, in consequence of this assumption, he regards such qualities as colour, taste, etc., as subjective sensory impressions to which atoms in different arrangements correspond objectively, he lays the foundation of a distinction between primary and secondary qualities which has not been rightly appreciated until modern days.