Ancient Greek Philosophy and the Scientific Method

Every explanation requires at least one assumption to be made, and the validity of the explanation of any observable fact is contingent upon three things:

1. the truth of the assumptions that are made,
2. the existence of a logical connection between the assumptions and the facts they are intended to explain, and
3. the consistency of all possible consequences of the assumptions with the observable facts.

Aristotle’s (384—322 BCE) physics relied heavily on the first two of these, but was lacking in the third. This isn’t because he was lazy or inept, but because there was more work to be done in order to make significant advances beyond Aristotle’s physics than any person could do in a lifetime. It took 2000 years worth of thought, detailed observation, and finally the invention of the telescope, to show that the facts are not all consistent with Aristotle’s assumptions, and that they are therefore wrong.

Aristotle’s theory was influenced by the Greek philosophers who came before him. The above three points simply mean that there is a basic cause (i. and ii.) of everything that happens (iii.). Even this explanatory framework may not be true: it is an assumption, and one that people have not always made. The first Western philosopher to teach that the Universe is rational and that the reasons why it works the way it does, was Thales of Miletus (c. 624—c.546 BCE). In contrast to earlier philosophers who thought the mysteries of the world were beyond human understanding, Thales and his followers believed mysteries were mysteries because they were unknown, not unknowable.

Thales is often regarded as the first philosopher in the Greek tradition—indeed, this was Aristotle’s opinion. Following Thales, there were a number of philosophers who influenced Aristotle, but unfortunately almost all of their work as been lost. Only fragments of the writings of every philosopher before Plato (424—347 BCE) have survived, where brief excerpts have been found quoted in some later work. However, enough has survived that we can now piece together a brief history of the development of influential ideas between Thales and Aristotle.

Following Thales, the next to influence the growth of the new scientific attitude was Pythagoras (c. 570—c.475 BCE). Even relative to the other philosophers prior to Plato, very little is known about Pythagoras. We do know that he and his followers discovered a number of mathematical relations that seem to be connected to nature, and therefore held that the Universe was governed by basic geometrical and mathematical relations; that the universe Thales said could be understood is essentially mathematical. Having discovered that musical pitch was related to the length of plucked strings, they held that musical principles governed, e.g., celestial motion. In module 3, we will see that the belief that the cosmos was not only mathematical, but musical, had a great influence on Kepler.

Heraclitus of Ephesus (c. 535—c. 435 BCE) is famous for a few reasons, not the least of which being that his writing was extremely obscure. He believed in a unity of opposites, that “the road up and the road down are the same thing.” More important, however, was his idea that everything flows and “you cannot step twice into the same stream”—meaning that each time you step down, the water that was there before has gone and there is new water flowing past. The credo “panta rhei,” which literally means “everything flows,” had a great influence on subsequent philosophy about the nature of time, which in turn led to the development of the hypothetical-deductive method, which we will discuss shortly.

Before discussing the process of deduction and how it fits into the scientific method, there is one further contribution of Heraclitus’ that is important to mention, as it also had great bearing on the subsequent history of astronomy. Heraclitus is the first philosopher we know to have lamented the fact that the causes of things that we commonly observe are not all obvious, and that without careful consideration people (who he referred to as barbarians) often infer incorrectly. This is one of the greatest problems in theoretical physics, and we deal with it by determining those principles—the basic causes—which lead to the most accurate description of observables. The problem was recognised by all later philosophers, and was famously illustrated through Plato’s Cave Allegory in The Republic.

After Heraclitus, Parmenides of Elea (c. 515—c. 460 BCE) approached the problem of the apparent passage of time by developing the method of logical deduction. The deductive process works by taking something as given and determining its consequences; “if A, then B” is its basic structure. In Parmenides’ case, his basic assumption was that everything that is is, and everything that is not isn’t—a pretty reasonable assumption to make. He went on from there to argue that only substantial things are, and that empty space—which is nothing but a receptacle for things that are, and not a thing itself—is not. He then said that everything that is is continuous matter, which obviously cannot move because there would be nowhere for it to go, i.e. since everywhere else is something else. Therefore, he concluded that motion and the apparent passage of time are illusions.

You should begin to see how one can easily paint oneself into a corner through logical deduction—and you wouldn’t be the first! Parmenides believed in the truth of his conclusions, but others did not. Most famously, the ancient Atomists, Leucippus (5th century BCE) and Democritus (c. 460—c. 370 BCE) assumed, in response to Parmenides, that what-is-not actually is, and they called it the Void. The Atomists maintained that all matter is composed of different-shaped atoms that fit together, often with empty space in between, and that these atoms all freely move within the Void. The Atomists’ theory bears some resemblance to the world of modern physics, in which it is understood that 65 billion subatomic particles called neutrinos emitted by the Sun pass through an area of 1 cm² every second at the Earth—and that the only reason other particles such as photons and electrons do not pass through as well is because they interact electromagnetically. Apparently solid objects, such as the Earth, are mostly empty space.

Modern physics tells us that matter is made up of particles too small for the eye to see, just as the Atomists proposed, but we’ve actually gone a step further in finding that if these particles only interacted by running into one another like billiard balls, they would almost never interact. In terms of foresight, the Atomists may be said to have come closer than any of the other ancient Greek philosophers to a picture of the Universe that is like the one described by modern physics. However, perhaps their greatest contribution was to the scientific method itself, as they realised that the assumptions one makes in constructing a particular theory are not strictly true—even when they’re as obvious as “what is is, and what isn’t isn’t”—but are hypotheses chosen as the basis of deductions, the logical consequences of which must be consistent with the evidence. This is the hypothetical-deductive method of science, which was mentioned above.

Plato, who held Parmenides in high regard, was said to have wished he could burn all the writings by Democritus that he could collect. Indeed, while a large number of Plato’s works have survived to this day, only fragments of Democritus’ writings still exist. One of the more famous ones expresses the same problem with interpreting observational data that was recognised by Heraclitus and Plato: “Of a truth we know nothing, for truth is in a well [an abyss]”(Diogenes Laertius, Lives of Eminent Philosophers, IX, 72).

And so it was that the ancient Greek philosophers, who believed the Universe to be basically knowable, and explainable from basic principles whose consequences should be the observable world around us, came to realise that the basic principles of any theory must be hypotheses that can never be definitively asserted, as any number of different principles could be causes of the observable facts.