While many other observers and thinkers had laid the groundwork for science, Thales (circa 624 B.C.E-ca 547 B.C.E.), the best known of the earliest Greek philosophers, made the first steps toward a new, more objective approach to finding out about the world. He posed a very basic question: "What is the world made of? " Many others had asked the same question before him, but Thales based his answer strictly on what he had observed and what he could reason out-not on imaginative stories about the gods or the supernatural. He proposed water as the single substance from which everything in the world was made and developed a model of the universe with Earth as a flat disk floating in water.

Like most of the great Greek philosophers, Thales had an influence on others around him. His two best-known followers, though there were undoubtedly others who attained less renown, were Anaximander and Anaximenes. Both were also from Miletus (located on the southern coast of present-day Turkey) and so, like Thales, were members of the Milesian School. Much more is known about Anaximander than about Anaximenes, probably because Anaximander, who was born sometime around 610 B.C.E, ambitiously attempted to write a comprehensive history of the universe. As would later happen between another teacher-student pair of philosophers, Plato and Aristotle, Anaximander disagreed with his teacher despite his respect for him. He doubted that the world and all its contents could be made of water and proposed instead a formless and unobservable substance he called "apeiron" that was the source of all matter.

Anaximander's most important contributions, though, were in other areas. Although he did not accept that water was the prime element, he did believe that all life originated in the sea, and he was thus one of the first to conceive of this important idea. Anaximander is credited with drawing up the first world map of the Greeks and also with recognizing that Earth's surface was curved.He believed, though, that the shape of Earth was that of a cylinder rather than the sphere that later Greek philosophers would conjecture. Anaximander, observing the motions of the heavens around the polestar, was probably the first of the Greek philosophers to picture the sky as sphere completely surrounding Earth-an idea that, elaborated upon later, would prevail until the advent of the Scientific Revolution in the seventeenth century.

Unfortunately, most of Anaximander's written history of the universe was lost, and only a few fragments survive today. Little is known about his other ideas. Unfortunately, too, most of the written work for Anaximenes, who may have been Anaximander's pupil, has also been lost. All we can say for certain about Anaximenes, who was probably born around 560 BCE, is that following in the tradition of Anaximander, he also disagreed with his mentor. The world, according to Anaximenes, was not composed of either water or apeiron, but air itself was the fundamental element of the universe. Compressed, it became water and earth, and when rarefied or thinned out, it heated up to become fire. Anaximenes may have also been the first to study rainbows and speculate upon their natural rather than supernatural cause.

With the door opened by Thales and the other early philosophers of Milestus, Greek thinkers began to speculate about the nature of the universe. This exciting burst of intellectual activity was for the most part purely creative. The Greeks, from Thales to Plato and Aristotle, were philosophers and not scientists in today's sense. It is possible for anyone to create "ideas" about the nature and structure of the universe, for instance, and many times these ideas can be so consistent and elaborately structured, or just so apparently obvious, that they can be persuasive to many people. A scientific theory about the universe, however, demands much more than the various observations and analogies that were woven together to form systems of reasoning, carefully constructed as they were, that would eventually culminate in Aristotle's model of the world and the universe. Without experimentation and objective, critical testing of their theories, the best these thinkers could hope to achieve was some internally consistent speculation that covered all the bases and satisfied the demands of reason.

虽然有很多观察家和思想家已经为科学研究打下了基础，但泰利斯（大约公元前624年至大约公元前547年），早期希腊哲学家中最著名的哲学家，第一次使用一种更加客观的新方法来认识世界。他提出了一个非常基础的问题：世界是由什么组成的？在他之前，有很多人问过同样的问题，但是泰利斯的答案完全根基于他所观察到的以及他能推理出的东西，并不是那些凭空想象出来的有关神或超自然力量的故事。他认为水是创造出世间万事万物的唯一物质，并且他还建立了一个宇宙模型，其中地球是一个扁平的圆盘，漂浮在水中。 与大部分伟大的希腊哲学家一样，泰利斯影响着他周围的人。毫无疑问，大部分追随者都没那么有名，但最著名的两位是阿那克西曼德和阿那克西米尼。两人都来自米利都（位于现在土耳其南部海岸），因此就和泰利斯一样是米利都学派的成员。阿那克西曼德比阿那克西米尼更出名，可能是因为大约生于公元前610年某一天的阿那克西曼德雄心勃勃地尝试描述宇宙浩瀚的通史。正如后来发生在另一对哲学家师生（柏拉图和亚里士多德）身上的事情一样，阿那克西曼德不同意他老师的观点，尽管他非常尊重老师。他不相信“整个世界以及它所包含的所有东西都是由水构成的”，反而认为是一种没有形状的看不见的物质（他称之为“阿派朗”）构成了世间万物。 但是阿那克西曼德最重要的贡献是在别的领域。虽然他不承认水是最基本元素，但他的确相信所有生命都源自于海洋，因此他也是第一批持有这一重要概念的人之一。阿那克西曼德被誉为是第一个画出世界地图的希腊人，还发觉了地球表面是弯曲的。不过他认为地球的形状是圆柱形，并不是后来希腊哲学家们推测的那样。阿那克西曼德观察发现了北极星周围天体运动的轨迹，他极有可能是希腊哲学家中描绘出宇宙中的天体完全围绕着地球这样画面的第一人，这一观点随后不断发展盛行，直到17世纪科学革命的到来。 不幸的是，阿那克西曼德所书写的宇宙历史大部分译遗失，如今也只有一些碎片幸存下来。他别的观点也已无法知晓。同样不幸的是，阿那克西米尼的大部分作品也已经遗失了，他也可能是阿那克西曼德的学生。关于阿那克西米尼，我们能确定的就是他可能出生于约公元前560年，追寻着阿那克西曼德的传统，同样也与自己导师的观念不一致。按照阿那克西米尼的说法，世界既不是由水构成，也不是由阿派朗构成，空气本身才是宇宙中的的基础元素。空气被压缩就变成了水和土；如果被稀释，它就加热变成火。阿那克西米尼可能也是第一个研究彩虹的人，而且研究的是导致彩虹形成的自然因素而不是超自然原因。 随着泰利斯和其他早期米利都学派的哲学家打开新世界的大门，希腊思想家开始思考世界的本质。这些智力活动激烈涌现，在大多数情况下是绝对具有创造性的。这些希腊人，从泰利斯到柏拉图和亚里士多德，都是哲学家，并不是今天意义上的科学家。比如，任何人创造了关于自然和宇宙结构的观点，多次表述这些观点，它们始终如一并且结构精巧，或者仅仅是看上去很有道理，那么它们就能说服大部分人，这是很有可能发生的。但是，关于宇宙的科学理论，设置相关论证体系的要求远多于各种各样被我们编织在一起的观察和类比，论证体系的设置必须非常谨慎，最终可能以亚里士多德的世界和宇宙模型告终。理论如果没有进行实验和客观严谨的测试，那么这些思想家有可能获得的最多就是内部一致的猜想，它们不仅包含一切理论基础，还可以满足逻辑要求。