For such a development we are not, of course, forced to assume a foreign influence: mere progress in refinement and in mental activity could bring it about; yet none the less it is probable that foreign influence did quicken the process. It is true that from the beginnings of the literary period Greek thought played with a certain freedom on myth, partly perhaps because the traditions visibly came from various races, and there was no strong priesthood to ossify them. After Homer and Hesiod, men looked back to those poets as shaping theology to their own minds. [452] But all custom is conservative, and Pindar's mind had that general cast. On the other hand, external influence was forthcoming. The period of Pindar and Æschylus [525-455 B.C.] follows on one in which Greek thought, stimulated on all sides, had taken the first great stride in its advance beyond all antiquity. Egypt had been fully thrown open to the Greeks in the reign of Psammetichos [453] (650 B.C.); and a great historian, who contends that the "sheer inherent and expansive force" of "the" Greek intellect, "aided but by no means either impressed or provoked from without," was the true cause, yet concedes that intercourse with Egypt "enlarged the range of their thoughts and observations, while it also imparted to them that vein of mysticism which overgrew the primitive simplicity of the Homeric religion," and that from Asia Minor in turn they had derived "musical instruments and new laws of rhythm and melody," as well as "violent and maddening religious rites." [454] And others making similar à priori claims for the Greek intelligence are forced likewise to admit that the mental transition between Homer and Herodotos cannot be explained save in terms of "the influence of other creeds, and the necessary operation of altered circumstances and relations." [455] In the Persae of Æschylus we even catch a glimpse of direct contact with foreign skepticism; [456] and again in the Agamemnon there is a reference to some impious one who denied that the Gods deigned to have care of mortals. [457] It seems unwarrantable to read as "ridicule of popular polytheism" the passage in the same tragedy: [458] "Zeus, whosoever he be; if this name be well-pleasing to himself in invocation, by this do I name him." It may more fitly be read [459] as an echo of the saying of Herakleitos that "the Wise [= the Logos?] is unwilling and willing to be called by the name of Zeus." [460] But in the poet's thought, as revealed in the Prometheus, and in the Agamemnon on the theme of the sacrifice of Iphigeneia, there has occurred an ethical judgment of the older creeds, an approach to pantheism, a rejection of anthropomorphism, and a growth of pessimism that tells of their final insufficiency.