What is reality? Author Philip K. Dick offers a succinct definition: "Reality is that which, when you stop believing in it, doesn't go away." It's a solid starting point — even if you don't believe in gravity, you'll still fall down if you trip. But this definition doesn't account for a universe of hidden interactions and seemingly irreconcilable operations, one that gets stranger and stranger the more we poke and prod.

And poke and prod we do, eternally. The quest to understand reality dates back millennia, starting with heavenly explanations for natural phenomena that steadily drifted toward more observation-driven assessments and eventually moved into the realm of intellectual theory-crafting.

But are we any closer to a complete explanation? Physicist and Nobel laureate Frank Wilczek thinks so, offering an account of "fundamental lessons we can learn from the study of the physical world" in his book, Fundamentals: Ten Keys to Reality. For Wilczek, our universe is composed of just a few ingredients governed by similarly few laws, but with minor differences in distribution or condition driving vastly different outcomes.

"The world is simple and complex, logical and weird, lawful and chaotic," he writes.

His work also does what great books do — create more questions than answers. Let's see what we can do about that as we dive into the fundamental and frustrating concept of reality with a look at where we've been, what we've discovered and what comes next.

Get Me a Philosopher, STAT!

Lacking scientific specifics and given the pervasive oddity of the universe, discussions of reality have often been confined to the realm of philosophy, which afforded the ability to swing for the metaphysical fences and — perhaps more importantly — explore human nature itself. Consider the 1892 paper "What Is Reality?" by philosopher David G. Richie. In it, he suggests that the ultimate reality isn't physical law or observable phenomena but rather the "potentiality" of thought itself.

"Unfortunately," he notes, "the uncritical metaphysics of the ordinary and of the scientific understanding do not generally take the notion of potentiality quite seriously. Hence it is necessary to take the longer route of philosophical criticism."

It's an intriguing supposition, and one that reflects a substantive portion of the human experience. The problem? It also reinforces the human-centric nature of philosophy itself: the efforts we take to ensure we don't feel too small given the scale of the universe and our place in it.

But to understand reality, we need to think bigger.

Fours Majeure

The first frameworks of human understanding of the universe relied on divine action. Thunderstorms? The gods are getting angry. Poor harvest? Probably the work of an offended or mischievous deity. Eclipse? Something has gone very, very wrong in the vaults of heaven.

And then came the Greeks, who proposed a new way of looking at the world. They defined four fundamental elements — earth, fire, air and water — and suggested that all matter was made of these elements in differing proportions. This quadratic quantity didn't last long, however. Aristotle soon added the element of "aether" that supposedly held the stars and planets. Early scientific experiments, such as those conducted by Empedocles, helped to differentiate between air and aether. By inverting a bowl filled with air in water, he showed that the air didn't simply vanish, suggesting that it a) wasn't aether and b) was actually something.

Worth noting? While the Greeks often get credit, they weren't alone in their elemental assertion — the ancient Chinese culture came to similar conclusions, as did those of Persia and Japan, according to The Five Elements in the Literary Heritage of Iran and Japan: A Comparative Study.

While the four-element framework was simplistic and eventually gave way to a much more precise understanding of chemical components and object interactions, it represented one of the first forays into a physical rather than philosophical understanding of reality.

He Ain't Heavy, He's Fundamental

No discussion of our physical universe is complete without a nod to Isaac Newton. While the apple story is almost certainly apocryphal, the fact remains that Newton suggested something no one had ever thought of: gravity.

It seems strange now, but in the late 17th century there was no concept of gravity, no universal law that explained why we don't simply float away into the aether at random. In his Philosophiae Naturalis Principia Mathematica, Newton laid out his case for gravity, suggesting that all objects exerted this attractive force on one another — and that the more mass they had, the bigger the force. This notion was incredibly handy, because it let scientists calculate the trajectory of thrown objects, account for the moon's orbit around the Earth and quantify the interplanetary relationships in our solar system. However, Newton did run into trouble when it came to explaining the mechanism of gravity, arguing that while it "must be caused by an agent acting constantly according to certain laws," he was uncertain exactly how this agent operated.

Lack of complete understanding aside, Newton's work paved the way for one of the biggest shifts in our comprehension of reality: the four fundamental forces, which help to explain almost everything that happens in our universe. As Space.com outlines, they are:

Throwing Curveballs

And then there's Albert Einstein. It seems he's always there, lurking in the science shadows — and he plays a critical role in our understanding of reality thanks to his theory of relativity.

Einstein's work was predicated in part by Niels Bohr's concept of complementarity, which he used to help explain the results of Thomas Young's Double Slit Experiment. This experiment showed that light had the properties of both particle and wave. Complementarity suggests that this wave/particle duality are both accurate descriptions of the same reality that "complement" rather than oppose one another — instead, human limitations in observation and interpretation make these results appear irreconcilable.

The theory of relativity expanded this concept to the universe at scale, suggesting that any measurements involving time and space are relative to both the location and motion of the observer. This is especially relevant to gravity, which Einstein suggested was not an instantaneous, attractive force between two objects but rather a curvature of spacetime determined by an object's mass. The bigger the object, the bigger the curve and the more substantive the gravitational force. So substantive, in fact, that observers close to a massive object would find time itself slowing down relative to the passage of time experienced by more distant observers.

Einstein's curvature theory, while significantly more complex, has also been consistently accurate in describing the movement of light around massive objects, including planets, stars and even black holes. And it has fundamentally changed the way we view the nature of reality and the universe.

Discovering the Real Deal: A History of Trial and Error

So, what does this continual conversation around the nature of reality suggest? That trial and error are essential to the process of understanding.

It makes sense, then, that discussions of reality have historically been relegated to philosophers. While scientists recognize the value of revising theories based on new evidence, the seeming lack of evidence for many fundamental forces has provided an ever-frustrating framework. Even Einstein — absolute out-of-the-box thinker that he was — didn't believe that gravitational waves caused by accelerating mass would ever be detected. But on September 14, 2015, all the Laser Interferometer Gravitational-Wave Observatory (LIGO) captured evidence of gravitational waves produced when two black holes collided, according to Science Alert. Since they were 1.3 billion light-years away, the signal took some time to arrive and was perilously tiny when it finally crossed our paths.

And this is hardly the end of our efforts to understand reality. Consider the ongoing discussions about dark matter, dark energy, string theory, quantum mechanics and worm holes — just when we think we've unlocked the secrets of how the universe works, it turns out we've only peeled back another layer, and what's inside makes absolutely no sense.

Strangely enough, it's the very unpredictability and unwillingness of the universe to give up its secrets that keeps us so obsessed. It's our nature; we're easily adaptable, even more easily bored and constantly looking for new challenges, new opportunities and new questions to explore. Asking "what is reality?" — over and over and over and over again — not only gives us something to do with these big brains of ours, but it also helps us understand both our universe and ourselves just a little bit better.

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What Is Reality? Your Quick-Start Guide to the Nature of the Universe

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