Humans are remarkable creatures. This one species has explored its own world and ventured into space. We've shown compassion toward one another in times of need. We've waged wars that spanned the globe, killing millions of our own kind for various reasons. And we've used the gray matter stored in our skulls to observe the world around us and determine how stuff works.
Making a list of amazing intellectual achievements is both easy and difficult. It's easy because humans have been around for a long time and we've achieved many things. It's difficult because the list of candidates is enormous -- how do you determine which ones make it in?
This list isn't ranked in any particular order. It's not an exhaustive account of the best of the best. Rather, it's a selection of achievements that have shaped how we perceive and understand the world.
10: Writing
It's difficult to stress exactly how important the invention of writing has been for the human race. Before writing, knowledge had to pass directly from mentor to student. Gaining wisdom meant finding the right teacher and studying for years to understand and build upon any information you could glean.
Writing is something ancient human civilizations developed independently of one another. The Sumerians, who lived in Mesopotamia, seemed to have been the first to invent writing around 3500 B.C. [source: University of Chicago]. Writing allowed humans to pass knowledge, wisdom and stories to future generations. With writing, humans could build upon what they learned much more quickly and efficiently.
Not everyone was a fan of writing. The Greek philosopher Socrates believed writing to be a dangerous invention. According to Plato, Socrates said that writing would do two things: First, it would hurt our ability to remember information -- instead of committing knowledge to our brains we could rely on the written word. Second, Socrates worried that people would assume they have knowledge simply by possessing the right records [source: The Phaedrus]. Owning a book about geometry doesn't necessarily mean you understand theorems and properties.
9: Mathematics
The concept of mathematics is a phenomenal achievement. Mathematics is the science of numbers and how they interrelate. It's how we perform operations with numbers to make calculations. We use mathematics in everything from mundane activities like balancing a checkbook to theorizing how massive systems such as entire galaxies move through the cosmos. And mathematics can be abstract -- pure mathematics is a pursuit without a goal for application in the real world, though the results of discoveries in pure mathematics can lead to real-world applications.
The concept of numbers dates further back than writing. We have records of ancient people recording numbers in various ways dating back to 30,000 B.C. By 1950 B.C., humans were inventing and solving quadratic equations. Ancient Babylonians developed complex theories for geometry and algebra [source: The University of St. Andrews, Scotland]. These weren't just philosophies -- ancient civilizations used mathematics the way we do today to explain the way things work.
Without mathematics, it would be virtually impossible for us to build a deep, lasting understanding of the universe. Mathematics allows us to take measurements and make predictions based upon observations. It's truly an amazing intellectual achievement.
8: The Laws of Logic
The Greeks were thinking a lot back in the fifth century B.C. They thought about many things, including how to think. Legend has it that a philosopher by the name of Parmenides thought so hard that he invented logic in a burst of inspiration. That legend says Parmenides was living on a rock in Egypt at the time -- presumably he didn't have much else to do.
In reality, the laws of logic -- or how to frame an argument -- do originate in fifth century B.C. Greece. The original purpose of logic was to build a case for a particular argument or point of view. You use logic to support your argument and refute your opponent's arguments. If logic shows your argument has faults, you must address those faults or admit your argument is invalid.
An argument is a series of propositions. All but one of the propositions support the truth of the final proposition, which is the conclusion of the argument. An argument is sound if the conclusion is true assuming the premises supporting it are also true. The laws of logic led to other intellectual achievements like the development of the scientific method.
7: Philosophy
Philosophy is a discipline that defies easy explanation. You can think of philosophy as a means to gain knowledge about aspects of our world that defy empirical observation. In other words, it's an attempt to learn and know things without being able to measure them in a scientific way.
Within philosophy are several core concepts, including ethics, aesthetics, logic and epistemology -- the theory of knowledge. There's another category that, like philosophy itself, isn't an easy concept: metaphysics. The term comes to us from a collection of works by Aristotle, though Aristotle himself didn't coin the term. Metaphysics concerns itself with big questions. How big? "What is the meaning of being?" is a good example.
Because matters of philosophy tend to fall outside the realm of empirical study, it's next to impossible to support philosophical arguments with measurable facts. Instead, you must rely upon logic to build arguments to support a philosophical position.
Some things we think of as science might really fall under the umbrella of philosophy. For example, string theory attempts to explain the universe as being built upon elementary strings that vibrate in different ways. But it's impossible to measure or observe these strings -- we can only create conjecture based on complicated mathematical formulas. This has led some scientists to refer to string theory as a philosophy, not a scientific theory.
6: Newton's Laws
You've likely heard the apocryphal tale of Isaac Newton sitting beneath an apple tree in an orchard when he was truck on the head with a falling apple, which led him to consider the force of gravity. In truth, Newton formulated his theory about gravitational forces during a period in which several other great thinkers were also tackling the concept.
Decades before Newton's publication of "Principia," the book that explained the law of gravity, an astronomer named Kepler proposed laws of planetary motion. Kepler had observed the paths planets take and may have been the first scientist to introduce the term "orbit" to describe them. Gravity is the force that explains these orbital paths.
Newton proposed that there was a force of attractiveness -- everything that has mass is attracted to everything else. The strength of that attractive force depends upon the mass of the objects and the distance between them. But Newton didn't just make observations about the force of gravity.
Newton proposed three laws of motion. The first was that objects in motion tend to stay in motion while objects at rest tend to remain at rest. The second law applies to velocity and how it can change as you apply force to an object -- it explains how to calculate acceleration. His third law of motion is that for every action there's an equal but opposite reaction.
Newton's laws provide the foundation for many concepts in classical physics and shaped scientific thought for centuries. They're impressive achievements, even if the apple story is just an appealing fib.
5: Germ Theory
Before the late 17th century, we believed that some creatures appeared through spontaneous generation. That means organic life would spring from inorganic material -- a notion we now recognize as being false. But at the time, philosophers were unable to see life on a microscopic level and so assumed that tiny creatures or groups of creatures sprang into being somehow.
Anton van Leeuwenhoek helped dispel that notion. He was a Dutch merchant who built a simple microscope and observed tiny organisms that were invisible to the naked eye. Two centuries later, Louis Pasteur proved that these organisms are present in the air and don't spontaneously generate. Through the work of Pasteur, Joseph Lister, Robert Koch, Ignaz Semmelweis and others in the 19th century, we learned that tiny organisms could cause disease. We also learned that sterilization could dramatically reduce the chances of infection during medical procedures.
These discoveries were revolutionary and led to new techniques and processes that have saved countless lives. Because of germ theory, doctors and scientists were able to develop antibiotics to fight off microbial infections. We also developed safer techniques for packaging and storing food.
4: Theory of Evolution
In the 1700s, Carl Linnaeus created a classification system to organize information about living things. This classification system is one we still use today, although biologists have modified it extensively over the years. But Linnaeus believed that species were fixed in form -- he thought that animal ancestors were the same as current animals dating back to the Garden of Eden [source: University of California Museum of Paleontology].
Not everyone agreed with Linnaeus's thoughts on species. A physician named Erasmus Darwin theorized that animals descended from common ancestors and changed over time. He also proposed that animals competed against each other within nature for resources and that had some impact on their traits. His grandson, Charles Darwin, built upon these initial conclusions. In 1859, Charles Darwin published his famous work, "On the Origin of the Species."
Darwin's theory was elegant: If a particular species has traits that improve its ability to survive -- what we call fitness -- it will have advantages over other species in that same environment. It will also pass those traits down to its offspring. Changes within species called mutations may prove beneficial or detrimental to the individual life forms. Beneficial mutations will help animals survive and may continue in future generations. Detrimental mutations would hurt an animal's chance to have offspring and so they wouldn't perpetuate.
Darwin's theories set the groundwork for discoveries in chromosomal inheritance. They also explained how new species could form over time -- they didn't all appear on Earth at the same time.
3: Laws of Thermodynamics
Science is all about discovering how the universe works. Some applications of science examine minute phenomena that explain the tiniest fraction of the universe. Other approaches attempt to determine consistencies that fundamentally explain the way the universe works. The laws of thermodynamics fall into the second category.
Developed over hundreds of years, the laws of thermodynamics really took form in the 19th century. That's when scientists like James Prescott Joule, William Thomson and Robert Clausius formulated theories about how energy works within the universe. In the early 20th century, Walther Nernst contributed theories about the relationship between energy, entropy and temperature. Other scientists have tweaked and added laws over the years.
The first law of thermodynamics is that energy -- the ability to create change or do work -- can't be created or destroyed. But energy can change from one form to another. For example, kinetic motion can change into heat. The second law concerns entropy and the fact that it's impossible for heat to transfer from a cooler body to a warmer body if that is the only transfer of energy within a system. The third law states that a perfect crystal of a given element has an entropy of zero as you approach the temperature of absolute zero. Absolute zero is the temperature in which there is no motion within a system.
There are other laws of thermodynamics, including a zeroth law concerning thermal equilibrium among three systems. These laws explain the fundamental relationships between different forms of energy within the universe.
2: Einstein's Theories of Relativity
Between 1905 and 1916, Albert Einstein seemed to turn the scientific world upside down. In that time, he proposed two major theories about how the universe works. The first was his theory of special relativity. The second was his theory of general relativity.
Special relativity focuses on inertial motion -- the motion of something moving in a straight line at a constant speed. According to this theory, the speed of light is like a universal speed limit -- nothing travels faster. Einstein also theorized that matter can approach but never equal or exceed the speed of light. It's this theory that gives us the famous equation of E=mc2. That means that energy equals the mass of an object multiplied by the square of the speed of light. A tiny particle of matter would convert into a phenomenal amount of energy, a fact that makes atomic weapons and nuclear power plants possible.
General relativity is all about gravity. Einstein's theory of special relativity showed that one of Newton's ideas on gravity couldn't possibly be true. Newton's law ascertained that the force of attraction between bodies is instantaneous, but Einstein's theory of special relativity said instantaneous propagation is impossible. Einstein had to work on a new theory of gravity to resolve the conflict.
Einstein used a thought experiment to show that within a closed system it's impossible to tell the difference between the effects of gravity and that of a change in acceleration. General relativity states that gravity isn't some invisible force of attraction but rather a consequence of the shape of space-time.
1: Quantum Mechanics
Even Einstein had trouble wrapping his head around quantum mechanics. That's because one of the concepts in quantum mechanics -- a science that describes how stuff works on the subatomic scale -- appears to violate special relativity. The concept involves entanglement.
First, you have to accept that the subatomic world behaves very differently from the world of classic physics. One key concept in quantum mechanics is that observing a state changes it. According to quantum mechanics, a spinning particle will spin in all possible states at the same time until someone observes it, at which point its spin will settle into a single state.
If you created a system with two entangled particles with opposite spins, the combined state of the two particles would be zero spin because they cancel each other out. Because the particles are entangled, they will always spin opposite to one another, but according to quantum mechanics, their spin exists in all possible states at the same time. If you were to separate the two particles and then observe the spin of one of them, you'd know the spin of the other. It would be the opposite of the first particle.
Einstein had a problem with that. If observing one particle caused it to instantaneously settle into a single spin state, and if it were entangled with a second particle on the other side of the universe, that would mean you'd change the state of a particle faster than light could travel. It violates special relativity.
It's tough luck for Einstein -- experiments support quantum mechanics. Somehow it's possible to affect the state of a particle faster than light can travel. And that's just one facet of quantum theory -- the more we learn, the more we have to adjust how we think of the universe.
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