In answer to such questions, the Chaldean thinkers, slightly before the rise of the Greek schools of philosophy, developed the idea of the seven heavens in their crystalline spheres encircling the earth as their center.[2] This conception seems to lie back of both the later Egyptian and Hebraic cosmologies, as well as of the Ptolemaic. Through the visits of Greek philosophers to Egyptian shores this conception helped to shape Greek thought and so indirectly affected western civilization. Thus our heritage in astronomical thought, as in many other lines, comes from the Greeks and the Romans reaching Europe (in part through Arabia and Spain), where it was shaped by the influence of the schools down to the close of the Middle Ages when men began anew to withstand authority in behalf of observation and were not afraid to follow whither their reason led them. [Footnote 2: Warren: 40. See "Calendar" in Hastings: _Ency. of Religion and Ethics_.] But not all Greek philosophers, it seems,[3] either knew or accepted the Babylonian cosmology.[4] According to Plutarch, though Thales (640?-546? B.C.) and later the Stoics believed the earth to be spherical in form, Anaximander (610-546? B.C.) thought it to be like a "smooth stony pillar," Anaximenes (6th cent.) like a "table." Beginning with the followers of Thales or perhaps Parmenides (?-500 B.C.), as Diogenes Laërtius claims,[5] a long line of Greek thinkers including Plato (428?-347? B.C.) and Aristotle (384-322 B.C.) placed the earth in the center of the universe. Whether Plato held that the earth "encircled" or "clung" around the axis is a disputed point;[6] but Aristotle claimed it was the fixed and immovable center around which swung the spherical universe with its heaven of fixed stars and its seven concentric circles of the planets kept in their places by their transparent crystalline spheres.[7] [Footnote 3: For a summary of recent researches, see the preface of Heath: _Aristarchus of Samos_. For further details, see Heath: _Op. cit._, and the writings of Kugler and Schiaparelli.] [Footnote 4: See Plutarch: _Moralia: De placitas Philosophorum_, Lib. I et II, (V. 264-277, 296-316).] [Footnote 5: Diogenes Laërtius: _De Vitis_, Lib. IX, c. 3 (252).] [Footnote 6: Plato: _Timæus_, sec. 39 (III, 459 in Jowett's translation).] [Footnote 7: Aristotle: _De Mundo_, c. 2 et 6 (III, 628 and 636).] The stars were an even greater problem. Anaximenes thought they were "fastened like nails" in a crystalline firmament, and others thought them to be "fiery plates of gold resembling pictures."[8] But if the heavens were solid, how could the brief presence of a comet be explained? [Footnote 8: Plutarch: _Op. cit._, Lib. III, c. 2 (V, 303-4).]