All this, however, does not alter the fact that the work of Protagoras posed the very question of the existence of the gods as a problem which might possibly be solved in the negative. He seems to have been the first to do this. That it could be done is significant of the age to which Protagoras belongs; that it was done was undoubtedly of great importance for the development of thought in wide circles. Prodicus of Ceos, also one of the most famous sophists, advanced the idea that the conceptions of the gods were originally associated with those things which were of use to humanity: sun and moon, rivers and springs, the products of the earth and the elements; therefore bread was identified with Demeter, wine with Dionysus, water with Poseidon, fire with Hephaestus. As a special instance he mentioned the worship of the Nile by the Egyptians. In Democritus, who was a slightly elder contemporary of Prodicus, we have already met with investigation into the origin of the conceptions of the gods. There is a close parallel between his handling of the subject and that of Prodicus, but at the same time a characteristic difference. Democritus was a naturalist, hence he took as his starting-point the natural phenomena commonly ascribed to the influence of the gods. Prodicus, on the other hand, started from the intellectual life of man. We learn that he had commenced to study synonyms, and that he was interested in the interpretation of the poets. Now he found that Homer occasionally simply substituted the name of Hephaestus for fire, and that other poets went even further on the same lines. Furthermore, while it was common knowledge to every Greek that certain natural objects, such as the heavenly bodies and the rivers, were regarded as divine and had names in common with their gods, this to Prodicus would be a specially attractive subject for speculation. It is plainly shown by his instances that it is linguistic observations of this kind which were the starting-point of his theory concerning the origin of the conceptions of the gods.