Have you ever opened up an old notebook or journal, and as you casually flipped through the pages felt like you were reading the thoughts of someone entirely unlike yourself? Often our past selves can seem like completely different people, and the thoughts we had when we were younger are not the same thoughts we have today.

Sometimes those foreign-yet-not ideas are absolute rubbish, meant only for the pages of the journals, and the deep corners of our memories, our present minds more advanced than those early ruminations. And sometimes, it’s the complete opposite: the notions – opinions, arguments, and beliefs – that we had when we were younger are sources of inspiration, insight, and new intuition. Our past selves can be our greatest teacher, but only if the opportunity is right.

The history of science is replete with examples of both kinds. Sometimes, ancient ideas simply get left behind. And sometimes, they linger in the background for centuries, even millennia, before modern scientists find a new piece of evidence, which is all it takes to rekindle ancient, long-dormant thoughts, and give them their chance to finally blossom.

Ideas like the ultimate nature of reality itself.

If you could somehow construct a super-microscope and peer into the tiniest crevices of existence, what would you find? Would you see teeming scores of miniscule particles, zooming, floating, and colliding throughout the spaces between? Or, would you find the constituents of matters endlessly divisible, a chaotic mixture of fluids that scale down to the infinitesimal?

It seems as if this question should have a simple, binary answer. Either the universe is composed of fundamentally indivisible chunks, or it’s not. Us and everything around us is either made of tiny, inscrutable atoms, or we are composed of fluid-like substances that encompass the entirety of existence without finer division.

In the 5^th century BCE, the Greek philosopher Leucippus and his student Democritus made a simple argument. Look at, say, a mountain. You can split that mountain in half, and get two smaller hills in its place. You can then divide one of those hills into two smaller bumps. Take a bump, and chop it in half. Keep going, chopping and halving and dividing as you please. Eventually, you’ll find yourself dealing with incredibly small and almost insignificant clumps of matter.

But, eventually, you’ll have to stop. Democritus argued that you can’t keep dividing matter to infinity, because at some point, you simply won’t have any matter to deal with. At some incredibly tiny scale, well below the limits of human perception, you’ll encounter the atom: the indivisible, uncuttable, fundamental unit of matter.

In the universe of Democritus, there were two ingredients: the atoms, and the void through which they moved. The atoms were necessarily so small they could not be discerned by the human eye, and they could combine in a variety of interesting and complicated ways to generate the variety of experiences that we see around us.

Democritus took this idea one step further, arguing that the only thing that could be said to be real were the atoms. They were, in other words, the totality of material existence. Everything else, from the horses in the stable to the stars in the sky, were mere temporary agglomerations of these fundamental atoms, and had no more intrinsic reality than a brief thought that exits our minds as quickly as it arrives.

These early Greek philosophers (and others who agreed with them) were not the only ones to arrive at atom-based conclusions. For example, In the 8^th century BCE, the Indian Vedic sage, Aruni, proposed that all matter was composed of tiny little particles, which he called kana. While superficially similar to the atoms of the Greeks (which, some scholars argue, is a result of significant diffusion and dialogue between the two cultures), Aruni arrived at his realisation from a completely different direction.

In Aruni’s line of thinking, there were several different types of kana, each with their own set of properties. The kana combined in various interesting and complicated ways, permeating the universe with a sense of order and organisation that it had previously lacked. Without the kana and their machinations, the materials of the universe would mix into an unrecognisable mass of confusion and fluidity. To create the differences between the air, the water, and the earth beneath our feet, the kana had to manifest themselves into their distinct entities. Without the kana, all would be chaos.

But alongside the atomist philosophers in ancient Greece were thinkers who argued the exact opposite. After all, the existence of atoms had two difficult hurdles. One, to have atoms, you must have a void for them to move around in, and how could nothing claim to have an existence? Two, how could the concept of souls and the mind be compatible with a philosophy of atoms clearly focused on the material only?

Anti-atomists claimed that instead of a multitude of invisible, teeming particles that assembled and reassembled into our material experience, there were simply fluid-like, continuous substances. Ultimately, these substances performed the same function; namely, arranging themselves into interesting and varied combinations. But these substances had a fundamentally different nature than atoms.

In these theories, the substances were independent, real entities that inhabit the universe (or are, in some cases, literally the entire universe). What we perceive to be independent objects, like you and me and rocks and trees, are merely temporary mixtures of those fundamental substances that have arisen, and formed themselves into something corporeal.

How many fundamental substances are there? Some claimed that there was one – literally, one. In this school, known as the monists, the entire universe was a single immense unmoving block, and all motion and change was an illusion. In contrast, with the dualist schools of European medieval Christianity, there were two fundamental substances: mind and body. Lastly, in the pluralist tradition favoured by Aristotle, there were four: fire, earth, water, and air. From these all else took shape and character.

Eastern scholars, whether through cultural diffusion or their own ingenious thoughts, came to similar conclusions. Many Buddhist schools embraced a substance-like theory of the universe, concluding that all formations and structures were impermanent and inherently unstable. Everything crumbles and gives way to dust, and the dust blows in the wind before settling to become part of yet another structure, repeating these cycles of making and unmaking constantly throughout time and creation. Thus, all material objects – ourselves included – lacked any constant core or ‘self’. All that exists are the fundamental substances of nature.

In the 7^th century, CE Eastern thinkers took this even further, creating a kind of synthesis between atomism and substance theories. Perhaps, atoms do exist, they said, but are ethereal and impermeant, constantly flashing in and out of existence. Their fleeting and constant change expresses itself in the objects of material existence, but they themselves exist independently of it.They arise out of the fundamental substances of the world and return back to it, a frothing chaotic mess that gives rise to reality.

While initially persuasive, the reasoning of Democritus, Aruni, and their atomist pals didn’t necessarily convince their peers. On the surface, the theory of atoms isn’t that crazy of an idea: that there’s a smallest possible thing. The problem is, though, unless you have this smallest possible thing pegged in a display case, there’s no serious reason to believe it, and you can go about your normal everyday life assuming that the stuff inside you and around you is just that – stuff – and isn’t necessarily made of anything smaller. It just…is.

Across cultures around the world (at least, the ones that we have access to in preserved written records), atomism largely fell out of favour. Without any way to experimentally test the ideas, a substance-like substrate to reality just made sense, and aligned with many other theological and philosophical conceptions of reality.

For example, in the West, Aristotle was a big anti-atomist, and medieval scholars had a certain fanboy-level regard for him, as his philosophy dovetailed nicely with Christian theology. In particular, atoms required the existence of a void, and voids were places where God could not exist, which flew in the face of the very concept of an all-present deity.

And, so, the arguments about the ultimate nature of reality largely languished for over a millennium. Even though discussions on the topic never really ceased, philosophers merely continued to refine old substance theories, debating the finer details and integrating them into larger philosophical and theological worldviews. However, starting in the 17^th century, philosophers began to introduce more and more atom-like ideas, finding it a more natural fit for a mechanistic (and eventually, scientific) view of the universe.

And then, in the early 19^th century, something different happened: an experimental result that propelled a full revival of the old atomist theories. This revival didn’t happen in an instant, and had rested on many centuries of growing awareness and discussion. But, while philosophers had toyed with the idea of atoms for some time, there still wasn’t much concrete to say on the subject, because we did not have the capacity to directly observe them.

The experiment that triggered the revival came via English scientist John Dalton. Dalton was studying chemical reactions, especially the kind where one substance could be broken down into separate, lighter substances (these lighter substances would eventually be given the name elements). In his work, he discovered a curious relationship, something we now call the law of multiple proportions. For example, one of the substances Dalton was playing with was tin oxide. One oxide of tin is a grey powder, and when he broke that down into tin and oxygen, for every 100 parts of tin, he would get 13.5 parts of oxygen. Another oxide of tin appears as a white powder, and when he broke that down, for every 100 parts of tin, he would get 27 parts of oxygen.

The ratio of 27 to13.5 simplifies to 2:1, and this suggested to Dalton that there was something fundamental and indivisible about oxygen. He experimented with all sorts of chemical combinations and found similar, simple ratios of numbers in their products. The only way this could work, he argued, was to revive the ancient atomist idea and say that the elements consisted of tiny, inseparable, uncuttable units – the atoms. Why else would we get these simple ratios?

While Dalton’s arguments were persuasive, as it was incredibly difficult to explain his experimental results via substances, it took almost an entire century for the idea of atom to finally click, and the person to do it was Albert Einstein himself.

In 1905, Einstein wrote a collection of papers that, one by one, revolutionised the entire field of physics. In one of those papers, he studied Brownian motion. In 1827, the English botanist Robert Brown found that if you dropped a pollen grain in some fluid, bits of dust floating off the pollen would jiggle around in random, unpredictable ways.

Einstein found an answer for this mysterious behaviour: atoms. If the fluid was made of incredibly tiny, invisible, indivisible particles, then the random motions of the particles would cause them to strike the dust grains, causing it to jiggle around. If instead, the fluid were totally continuous, then this motion wouldn’t happen. Einstein’s result was far and away accepted as the final clincher for the supremacy of atomic theory, first proposed by ancient Greek and Indian philosophers and, millennia later, using experimental evidence as a guide, cemented into scientific certainty.

But, this is not the end of the story.

The discovery of subatomic particles in the early 20^th century did not spell the end of atomism in Western thinking – it simply moved the goalposts. Sure, atoms were not as uncuttable as their names suggested, and with enough energy, we could separate them into protons, neutrons, and electrons. Even some of those were eventually found to not be fundamental, but the idea that “everything is ultimately made of tiny indivisible chunks” held sway. Today, modern physics features a zoo of particles with exotic names: quarks, leptons, neutrinos, and so on. From that menagerie and their countless interactions via the fundamental forces, the world of the micro- and macroscopic takes shape.

But, when scientists attempted to explain the behaviour of those subatomic particles, they had to invent a whole new branch of physics: quantum mechanics. And, in that theory, and the experiments that led to that theory, they found a remarkable discovery.

According to our best available evidence, it seems that atoms are not so atomic after all, and that fundamental particles are neither fundamental nor particles.

The modern view of physics eschews particles altogether, and instead, posits that fields are the fundamental aspects of reality. These fields suffuse all of space and time, and there is one field for every type of particle. Electrons get their own field, for example, while photons get another (which you may know as the electromagnetic field), and so on. Every cubic centimetre of space and every microsecond of time is soaked in these overlapping quantum fields.

These fields support waves, wiggles, and other excitations. Sometimes, these excitations are brief and temporary, while other times, they are long-lasting and capable of travelling long distances. When the latter case happens, we experience those excitations as a particle. In this picture, particles can be created and destroyed as easily and flippantly as the ocean can churn into a great raging storm, or settle into a placid calm.

We see here yet another revival, once again based on the evidence, and once again bringing to the present an ancient idea. This time, a revival of Aristotelean and Buddhist concepts of the fluidity of our material world, brought from the depths of ancient history, and applied to the latest thinking in quantum theory.

For ages, philosophers and mystics have inquired into the fundamental nature of reality. Schools of thought have come into fashion and gone out again; ideas have been shared and mixed from culture to culture. Science itself, modern as it is, remains a part of that grand tradition, inheriting the intellectual gifts of its forebearers.

While science brings new tools to bear – namely, a reliance on experimental evidence – when confronted with the results of those experiments scientists must find some way to explain them. And when they go searching for ideas, they dip from the same well of human intellect, finding resonance and significance in the writings of old. Our insatiable curiosity continues to circle around the same answers time and again, sharpening and refining them with each revival.