Plato (427-348 B.C.E.) and Aristotle (384-322 B.C.E.), both Greeks, are commonly considered as the greatest philosophers of all time. Their works continue to be studied with keen interest today, since they shaped a wide range of philosophical disciplines for many centuries ahead.
On Truth:
For Plato, the world is like a cave in which people perceive not the true form of reality but the mere shadows of objects that cast upon the cave wall. In this regard, we cannot rely on our senses to understand the Truth of things by examining the natural world. Similar to fundamental principles of mathematical proofs, the true forms of natural things exist in our minds. Hence, the true form of reality is contemplated rather than observed and only the philosopher comes to understand the truth of things.
Unlike Plato, Aristotle believed that the senses were needed in order to properly determine reality of the natural world. The systematic observation and analysis of the natural world in combination with logic is the road toward Truth. Such methodology of observation is what we would now call the "science".
