Like the Egyptians and Babylonians before them, the Greeks originally conceived the universe as mythic, capricious, ruled by gods and therefore impossible to predict. In the VIth Cent. BCE in Greek Ionia (present western Turkey) there emerges a new way of thinking alongside the mythic way. The Ionian philosophers’ explanations become naturalistic; the gods disappear as prime causes for all events. The world becomes orderly, predictable, a place where things behave according to their nature, no longer according to the rules or whims of the gods. We call this Natural Philosophy. To these Greeks, this represents a new way of looking at the world using logos, reasoning as an approach to understanding the universe, rather than mythos, myth. Logos is rational, searches for objective truth, and is the same for all. 

Mythos is the world of imagination, free from the tyranny of reason. It deals with things that cannot be verified, subjective problems and depends on beauty for its appeal and understanding of the world.
Mythic stories are basically stories that tell how one thing arose from another (like all the begats...). The problem with such a system is that because everything arises from something else, there is no beginning, and no end. Nothing is fundamental or basic. It is a narrative, a story, not a truth. Such a system can't be argued with, it makes no statements, and therefore is not open to critical review. You can't argue with a story that says that Aphrodite arose from the foam of the sea. You may or may not believe it, but you cannot comment on it critically, or discuss it logically.

The first famous name that stands out in this pageant of great greek thinkers is Thales, the natural philosopher who first applied logos (reasoning) to the natural world.

For Thales (and other later philosophers), all things in nature are simply modifications of some basic stuff, from which they arise and to which they go back to when they end or are destroyed. For Thales, it was water. What is important about this, is that this is a statement, an assertion of the way things are. Such a statement can be criticized and argued with. It can be contradicted or supported with examples, observations and with logic by showing contradictions. It is Thales that invented the notion of proof in mathematics, because the notion that things are self-evident is insufficient. A proof makes a statement logically unarguable. Thus Thales introduced a new way of dealing with the natural world, a new intellectual procedure. 

Thales saw the Earth like a giant cup floating in water, much like a ship. Earthquakes happen when there is motion in the water. The moon is earth-like (he does not explain why it does not fall, if like the earth  it is heavy) and shines from sunlight reflected off its surface. He had realized that eclipses occur only during full and new moons, and occur in patterns. He used this knowledge to make a prediction of a solar eclipse. When the eclipse happened, he became famous.  

As they pondered nature logically, the Ionians posed three fundamental questions that have remained with us since that time. 
First, what is the underlying nature of the world, of reality? 
Thales thought it was water, others air or fire. Between 450 and 400 BCE the atomists suggest that the cosmos is made of atoms that constantly recombine. Whatever the answers of individual philosophers, they all agreed that the universe originated from one single thing and, that it was some kind of matter. Still others such as the Pythagoreans (in the colonies of southern Italy) disagreed and thought that the underlying reality was numbers, not matter. 

Second, what is the nature of change? 
Some claimed that change was fundamental. Others such as Zeno, that change is but illusion (Zeno’s famous paradox). 

Third, what is the basis and nature of knowledge? 
How do we know anything? Is knowledge based on logic, on reasoning? Is it based on the senses? On inspiration? While no consensus emerged, this third question focused concerns on how we test our knowledge. 
 

It should be pointed out that Thales and the Ionians did not appear in an intellectual vacuum. It's not as if all the other Ionian Greeks were shackled by myth, and along comes Thales to show them how illogical they have been all this time. Consider what it means that he became famous by predicting an eclipse. It means that he lived in a society that valued, supported and rewarded intellectual activity and achievement, even though this activity had no practical application. And that is one of the enormous differences between the Greek culture and those of the Babylonian and Egyptians before them and the Romans after them. And perhaps that is something that today, we also share with the Greeks.