It seems that at the beginning of the third millennium of Western civilization, we are still faced with the same perplexing question as our ancestors: What do we really know about our world and ourselves? By “know”, I do not mean to have cognizance or awareness, but to perceive and intellectively grasp a clear and certain understanding. But what exactly is a clear and certain understanding?
The Roman scholar Pliny the Elder posited in the first century A.D. that the only certainty is that nothing is certain. In the 17th century, the French mathematician, physiologist, and philosopher René Descartes started with this idea as a premise (actually an equivalent proposition using the word doubt) and used logic to establish a foundation for human knowledge in a similar manner as Euclid established a basis for geometry. But in his exhaustive search for certainty, he created an intellectual impasse that has since divided the natural world into mental and physical realms -- the Cartesian duality of “mind” and “matter”.
In 1739, the Scottish philosopher David Hume declared two types of truth: “truths of reason” (1+1=2) and “matters of fact” (If I release this ball, it will fall). Hume argued that all knowledge of the physical world is independent of reason and consists of only sensory experience. For instance, we see objects fall all the time, and thereby become cognizant or aware that it is the custom or habit for objects to fall. It is a “matter of fact” that things fall. Any explanation of how and why things fall is a “truth of reason,” and reason does not have any cause-and-effect relationship with the event of the object falling. The “law” of gravity expounded in 1687 by the English mathematician and physicist Isaac Newton demonstrates mathematically how things fall, and it precisely describes our observations of falling objects. But this is a product of reason, a human concept or idea of the cause and effect, not the cause and effect itself. The ancient Greeks called this “saving the appearances” -- i.e. this is a “mental model” or “construct” that we can use to attempt to understand and communicate how things appear to us to be true and real.
The 5th-century B.C. Greek sophist Protagoras, whose works were destroyed in antiquity and whose ideas survive only through Plato’s writings that bear his name, refused to differentiate between sensory experience and reason by denying altogether any possibility of objective knowledge -- that is we cannot know the reality of material phenomena independently of the concepts derived from our senses, and all knowledge of reality becomes subjective by each individual’s unique ability to perceive and reason.