How the Scientific Method Works

Four Bodily Fluids Shaped Medicine For 2,000 Years From Hippocrates To the 1600s

The four humors system dominated Western medicine from the time of Hippocrates in the fifth and fourth centuries BCE until the 1600s. Hippocrates is credited with popularizing the system, but it likely originated from earlier traditions, possibly from India or Egypt, before being adopted and expanded by Greek thinkers such as Galen.

Four Humors: Blood, Phlegm, Black Bile, Yellow Bile—Fluids Representing Energy, Emotions, Vital Forces in the Body

The term "humor" comes from the Latin word for "fluid." The four humors—blood, phlegm, black bile, and yellow bile—were believed to be the principal fluids of the body, but they symbolized more than just physical substances. They were thought to represent broader aspects like energy, emotions, and vital forces within a person. For instance, one’s mood or personality could be linked to an excess or deficiency of a particular humor.

Humors' Qualities and Disease Believed Due to Imbalance

Each humor was attributed with specific qualities: hot or cold, wet or dry. Diseases were classified based on these qualities, such as a "cold and wet" illness like pneumonia, and were considered the result of humoral imbalances. Symptoms were mapped to one or more humors being either in excess or lacking, and even personality traits or emotional states were tied to the mix of humors present in the body.

Restoring Equilibrium Through Humor Regulation

Restoring the proper balance of the humors was thought to be the key to regaining health. Treatments aimed to bring this equilibrium, often using the "opposite" qualities to counteract the afflicted humor’s state. Common interventions included bloodletting, forced vomiting, and prescribing herbs that were believed to have the needed hot, cold, wet, or dry properties. Each person was thought to have a unique, preordained balance that needed maintenance for good health.

Credible and Longevity Theory

Framework Linking Human Creation Beliefs To Authoritative Medical Explanations

The four humors theory successfully linked existing philosophical beliefs about human creation and the natural world to medical explanations of health and illness. Its explanatory power was reinforced by its adoption into philosophical and medical traditions, making it a dominant system for understanding the body for centuries.

Apparent Validity From Consistent Practitioner Repetition Across Centuries

Much of the credibility of the system stemmed from repetition by respected figures like Hippocrates and Galen and from its continual use by generations of physicians. Because the theory was taught, cited, and practiced for so long, consistency itself was seen as proof of its validity, with each generation reinforcing acceptance.

Theory's Accurate Intuitions About Body's Balance Misunderstood by Practitioners

While modern science has shown the four humors theory to be incorrect, it did contain valuable intuitions. The concept of homeostasis—balancing internal bodily conditions—aligns with the humoral focus on equilibrium, even though the mechanism and details were misunders ...

Ancient Greek philosophers devised a range of models to explain the structure and motion of the cosmos. Their ideas shaped Western cosmology for over two millennia, with the models developing in complexity and influence as thinkers sought to rationalize their observations with philosophical ideals of order and perfection.

Eudoxus' Geocentrism: Earth as Universe Center With Circular Heavenly Body Orbits

Geocentric Model Seemed Correct to Earth Observers as Celestial Objects Moved With No Sign of Earth Moving

Eudoxus of Cnidus, who lived in the 4th century BCE, reinforced the long-standing notion that Earth was at the center of the universe. This geocentric model appeared reasonable to ancient observers because, from a human perspective standing on Earth, the stars and planets seemed to move around the planet nightly with no evidence that Earth itself was in motion. It was natural philosophy at the time to view humans as the center of creation, lending cultural support to geocentrism.

Eudoxus's Proposal: Circular Motion as Perfect Cosmic Movement, Circles as Sole Geometric Orbit Descriptors

Eudoxus formalized key assumptions about the cosmos: all celestial motions are circular and regular, the center of each celestial movement is also the center of its path, and the center of all motion is the center of the universe—Earth. The circle was considered the perfect shape, befitting the divine order attributed to the heavens. Eudoxus also relied on circular orbits because mathematical descriptions of non-circular motion were not yet available.

Eudoxus's System of Nested Celestial Shells

Building on earlier ideas, Eudoxus developed a model of nested celestial shells to account for the observed motions of the planets, sun, and moon. He proposed an intricate system of 27 concentric spheres or “shells,” each carrying a celestial object around the stationary Earth. Some shells were nested within others, allowing for the complex apparent paths of planets. When further discrepancies appeared between prediction and observation, later followers simply added more shells for refinement, rather than rethinking the entire approach.

Anaximenes Precedes Eudoxus: Celestial Bodies in Concentric Spheres Rotating Around Earth

Before Eudoxus, Anaximenes in the sixth century BCE described the cosmos as consisting of concentric celestial spheres enveloping the Earth. Each sphere rotated around Earth, carrying the celestial bodies with them. This "shell cosmology" provided the foundation for Eudoxus's approach and persisted for 2,000 years.

Shell cosmology’s influence was so strong that even Copernicus, who revolutionized astronomy by placing the sun at the center of the solar system, still maintained the idea of planets circulating within celestial shells. It wasn’t until the 17th century that Johannes Kepler disproved the shell model by showing that planetary orbits are elliptical, not circular, thus laying the foundation for modern astrophysics.

Pythagorean Philosophers' Heliocentric Model Proposed a Central Fire With Celestial Bodies Orbiting, Including Earth, Sun, Moon, and Five Planets

While geocentrism dominated, the Pythagorean school, including the philosopher Philolaus, introduced a radically different concept. They proposed that a “central fire”—not the Sun—sat at the true cent ...

The origins of our understanding of matter date back to ancient thinkers who sought to explain what everything is made of. Their ideas shaped natural philosophy for centuries and, despite their primitive methods, contain surprising insights that foreshadow modern science.

Four Elements Theory: Earth, Air, Fire, Water Constitute Matter, 6th Century BCE; Dominated Natural Philosophy Over a Thousand Years

The concept of four basic elements—earth, air, fire, and water—as the foundation of all substances dates back to the 6th century BCE. During this time, philosophers debated whether all matter derived from a single substance or a combination of more than one. Anaximenes, for example, asserted that air was the fundamental substance from which everything else derived. Later, Plato established the idea of four elements—earth, air, fire, and water—while Aristotle famously introduced a fifth element, ether, which he believed filled the heavens.

Wood Exhibits Multiple Elemental Properties

The four-element theory explained everyday observations by ascribing elements to various characteristics of substances. For example, wood was considered solid due to its "earth" content, floated because of its "air" element, and burned, which demonstrated the presence of "fire." This thinking allowed ancient philosophers to attribute observed properties to specific elemental combinations, suggesting that everything is made up of these elemental portions in different degrees.

Aristotle's Endorsement of the Four-Element Theory Made It Canonical for Centuries

Aristotle's authority cemented the four-element theory at the core of natural philosophy for more than a millennium. His endorsement ensured the theory’s dominance until the scientific revolution. Aristotle not only supported the idea of the four key elements but also claimed they could transform into one another, and that every region, even the space between objects, was filled—not with emptiness—but with one of the elements, especially air or ether. As a result, the idea of a void or vacuum was rejected for centuries.

Empedocles Introduced That Matter Consists Of Tiny, Invisible Building Blocks Combining to Create Substances, Prefiguring Modern Elemental Composition Understanding

Empedocles of Sicily, active in the 5th century BCE, progressed the understanding of matter by proposing that all things are composed of tiny, invisible building blocks. He described these as combinations of four fundamental elements, not just simple substances in and of themselves, but entities so small that they were imperceptible to human senses.

Empedocles' Four Elements as Basic Components

He argued that an object like a rock is not fundamentally "rock," but is built from unique mixes of these elemental components. Empedocles' insight prefigured our current understanding that substances are made from basic components combined in various ways.

Attractive Forces Called "Love" Bring Elements Together

Empedocles attributed the union of the elements to an attractive force he called "love," bringing them together to create all things. This force was a prescient conceptual ancestor of the physical forces (like electromagnetism) that scientists today recognize as binding matter at the atomic and molecular level.

Democritus's Theory: Substances Are Made Of Indivisible, Eternal Atoms in Free Space

Democritus later introduced the idea that matter could be divided only down to a fundamental, indivisible unit—what he termed "atomos," meaning "uncuttable" in Greek. According to his atomic theory, these atoms were eternal and existed in free space, what is now called a vacuum.

Democritus Foresaw an Indivisible Unit in Matter Upon Division

Democritus posited that if a substance like a rock were divided repeatedly, you would eventually reach a base unit that cannot be divided any further: the atom.

Vacuum Theory: Atoms Move Through Empty Space vs. Space Filled With Substance

A radical aspect of his theory was the assertion that atoms moved through empty space. This concept of the vacuum stood in direct opposition to the prevailing notion that all space was filled with some form of matter, as backed by Aristotle.

Democritus's Mistake: Asserting Each Matter Had Unique Atoms, Instead Of Recognizing Combinations of Basic Atomic Types Make all Substances

Democritus erred in his belief that each kind of substance—rock, rabbit, or log—possessed its own unique atom, rather than realizing that all substances could be built from different combinations of a ...

Spontaneous generation, the idea that life can arise directly from non-living material, stands as one of the three major biological theories in antiquity, alongside preformationism and epigenesis. For centuries, people debated how living things originated, giving rise to persistent myths and scientific investigation.

Spontaneous Generation, one of Three Ancient Biology Theories Alongside Preformationism and Epigenesis

In the ancient world, thinkers often subscribed to multiple explanations for the origin of life. The three main theories were spontaneous generation, preformationism, and epigenesis.

Spontaneous Generation Proposed by Jean-Baptiste Van Helmont

Spontaneous generation famously found support in the 17th century from Jean-Baptiste Van Helmont, who proposed outlandish recipes for generating life. He claimed that placing grain in a cellar would produce mice, or lining a brick mold with basil would spawn scorpions. These illustrations demonstrate the theory's widespread belief—that life could simply spring from inanimate matter.

Maggots On Rotting Meat Confirm Spontaneous Generation

Another common example was the belief that maggots spontaneously generated from rotting meat. The appearance of maggots on unattended meat seemed to confirm that life emerged without external influence. At the time, these explanations did not seem far-fetched due to the absence of microscopic knowledge.

Leeuwenhoek's 17th Century Microscope Fails to Settle Spontaneous Generation Debate

The development of the microscope by Antonie van Leeuwenhoek in the 17th century did not immediately end the spontaneous generation debate. Instead, it revealed a previously unseen world of microscopic creatures and added complexity.

Spontaneous Generation Proponents: Microscopic Creatures, Not Larger Organisms

With the ability to see tiny organisms, spontaneous generation proponents argued that while larger animals might not arise spontaneously, microscopic creatures certainly could. Their sudden appearance in previously sterile environments continued to be cited as evidence for spontaneous generation.

Flask Boiling Experiments Question Spontaneous Generation Evidence

Researchers conducted experiments to test spontaneous generation in microbes by sealing flasks of water, boiling them to sterilize, and leaving them for days. When microorganisms reappeared, supporters claimed it proved spontaneous generation. However, critics argued that the water was not boiled long enough, so the experiments couldn’t conclusively support the theory.

Redi Disproved Spontaneous Generation: Covered Meat With Muslin Prevented Maggots

Before the microscope contributed to the debate, Tuscan physician Francesco Redi performed a pivotal experiment. He showed that when rotting meat was covered with muslin, flies could not reach it and maggots failed to appear. This experiment demonstrated that life—at least in the case of maggots—came from external agents, not spontaneous decay.

Life Was Brought by External Agents, Not Spontaneously From Decay

Redi's work established that living creatures like maggots were not generated from non-living substances but resulted from eggs laid by flies. This finding was an early but important challenge to the theory of spontaneous generation.

Spontaneous Generation Debunked by Pasteur

The final blow to spontaneous generation came in 1860 from Louis Pasteur. He demonstrated precisely how to sterilize flasks, preventing airborne microbes from contaminating boiled solutions. Pasteur’s experiments showed that life did not arise in these sterile envi ...