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.
It was the great work of Descartes to exclude rigorously from science all explanations which were not scientifically verifiable; and the prevalence of materialism at certain epochs, as in the enlightenment of the 18th century and in the German philosophy of the middle, 9th, were occasioned by special need to vindicate the scientific position, in the former case against the Church, in the latter case against the pseudo-science of the Hegelian dialectic. The chief definite periods of materialism are the pre-Socratic and the post-Aristotelian in Greece, the 18th century in France, and in Germany the, 9th century from about 1850 to 1880.
The pre-Socratic thinkers were all primarily devoted to ontological research; but by the middle of the 5th century B.C. the conflict of their dogmatic systems had led some of the keenest minds to doubt the possibility of penetrating the secret of the physical universe.