Why does the second hallmark say “as simply as possible”?

Notice that the second hallmark says that we seek to explain observations “as simply as possible.” You might wonder exactly what this means. It is easiest to understand through example. Remember that Copernicus’s original Sun-centered model did not match the data noticeably better than Ptolemy’s Earth-centered model. Therefore, if scientists had judged this model solely on the accuracy of its predictions, they might have rejected it immediately. However, many scientists found elements of the Copernican model appealing, such as its simple explanation for apparent retrograde motion. This perceived simplicity made people consider the idea to be worth further investigation, which ultimately led to Kepler’s discovery of three simple laws that successfully explain planetary motion. (Note that in this context, “simple” means something that can be stated with relatively few words or mathematical equations; of course, understanding these words and equations may require years of study.)

We can’t be sure that nature will always be “simple” at its heart, but so far, every major model ever established through science has proven to be easy to understand, at least after you’ve learned the basic concepts and mathematics involved with it. Here are a few more examples:

• As we’ll discuss in the next section, almost everything we know about gravity — from how objects fall to the ground to how moons orbit planets and planets orbit stars — can be explained with a single, simple mathematical law.
• The theory of matter explains the many different types of substances we encounter as all being made up from a relatively small set of atoms, and these atoms are in turn made up from just three types of particle (protons, neutrons, and electrons).
• The theory of evolution is simple because it invokes a single basic idea — natural selection — to explain the diversity of life.
• If you study physics or engineering, you will learn that everything we know about electricity, magnetism, and light can be explained with just four mathematical equations (known as “Maxwell’s equations of electromagnetism”).

It is this history of success with simple models that makes scientists seek simplicity when looking to explain newly discovered phenomena. In fact, this idea is so ingrained in scientific work that it has its own name: It is called Occam’s razor , after a medieval scholar by the name of William of Occam (1285–1347).