Plato and Aristotle are the most famous Greek philosophers, having lived centuries before Christ. To understand them, it is important to review the thinking that preceeded them. Some of the earliest thinkers were struggling with the concept of reality; of what does everything consist? Some thought all was water (Thales), others said basic reality was the atom , or numbers, or reason , or that there were four elements (air, water, earth, and fire).
Then came a period of questioning. Heraclitus proposed that everything was in constant flux, that "you cannot step twice in the same river," because the water is constantly changing. This produced doubts about whether man could really know anything for sure. Protagoras considered the truth relative, that "man is the measure of all things." Gorgias concluded that certain knowledge and communication are not really possible.
It is in this context that Plato and Aristotle come on the scene, making an effort to find the way back to certainty of knowledge.
