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Scientific Theory

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Scientific Theory

This photo shows a girl and her parents. They are having a discussion. As you can see, the girl is rolling her eyes. What do you think her parents may have said that caused this reaction? Could it be they have just grounded her for some reason? That’s certainly one possibility, but without any other information to go on, it’s “just a theory.” In other words, it’s just a hunch or a guess. Theories are very important in science, but in science a theory is never a hunch or a guess. It is much more than that.  

Not “Just a Theory”

The term theory is used differently in science than it is used in everyday language. A scientific theory is a broad explanation that is widely accepted because it is supported by a great deal of evidence. Because it is so well supported, a scientific theory has a very good chance of being a correct explanation for events in nature. Because it is a broad explanation, it can explain many observations and pieces of evidence. In other words, it can help connect and make sense of many phenomena in the natural world.

Examples of Theories in Physical Science

A number of theories in science were first proposed many decades or even centuries ago, but they have withstood the test of time. An example of a physical science theory that has mainly withstood the test of time is Dalton’s atomic theory. John Dalton was a British chemist who lived in the late 1700s and early 1800s. Around 1800, he published his atomic theory, which is one of the most important theories in science. According to Dalton’s atomic theory, all substances consist of tiny particles called atoms. Furthermore, all the atoms of a given element are identical, whereas the atoms of different elements are always different. These parts of Dalton’s atomic theory are still accepted today, although some other details of his theory have since been disproven.

Dalton based his theory on many pieces of evidence. For example, he studied many substances called compounds. These are substances that consist of two or more different elements. Dalton determined that a given compound always consists of the same elements in exactly the same proportions, no matter how small the sample of the compound. This idea is illustrated for the compound water in the Figure below. Dalton concluded from this evidence that elements must be made up of tiny particles in order to always combine in the same specific proportions in any given compound.

Water is a compound that consists of the elements hydrogen (H) and oxygen (O). Like other compounds, the smallest particles of water are called molecules. Each molecule of water (H2O) contains two atoms of hydrogen and one atom of oxygen.

Q: Dalton thought that atoms are the smallest particles of matter. Scientists now know that atoms are composed of even smaller particles. Does this mean that the rest of Dalton’s atomic theory should be thrown out?

A: The discovery of particles smaller than atoms doesn’t mean that we should scrap the entire theory. Atoms are still known to be the smallest particles of elements that have the properties of the elements. Also, it is atoms—not particles of atoms—that combine in fixed proportions in compounds. Instead of throwing out Dalton’s theory, scientists have refined and expanded on it.

There are many other important physical science theories. Here are three more examples:

  • Einstein’s theory of gravity
  • Kinetic theory of matter
  • Wave-particle theory of light

Keep It Simple

The formation of scientific theories is generally guided by the law of parsimony. The word parsimony means “thriftiness.” The law of parsimony states that, when choosing between competing theories, you should select the theory that makes the fewest assumptions. In other words, the simpler theory is more likely to be correct. For example, you probably know that Earth and the other planets of our solar system orbit around the sun. But several centuries ago, it was believed that Earth is at the center of the solar system and the other planets orbit around Earth. While it is possible to explain the movement of planets according to this theory, the explanation is unnecessarily complex.

Q: Why do you think parsimony is an important characteristic of scientific theories?

A: The more assumptions that must be made to form a scientific theory, the more chances there are for the theory to be incorrect. If one assumption is wrong, so is the theory. Conversely, the theory that makes the fewest assumptions, assuming it is well supported by evidence, is most likely to be correct.


  • A scientific theory is a broad explanation that is widely accepted because it is supported by a great deal of evidence.
  • Examples of theories in physical science include Dalton’s atomic theory, Einstein’s theory of gravity, and the kinetic theory of matter.
  • The formation of scientific theories is generally guided by the law of parsimony. According to this law, the simplest of competing theories is most likely to be correct.


  • scientific theory: Broad explanation that is widely accepted because it is supported by a great deal of evidence.


Watch the first presentation by Dr. Eugenie Scott at this URL and then answer the questions below.

http://www.youtube.com/watch?v=ItxVLu8J_d0 (9:50)

  1. How does Dr. Scott define scientific theory?
  2. From most to least important in science, how would Dr. Scott rank the following concepts? theory, fact, law, hypothesis
  3. Based on the presentation, explain the importance of theories in science.


  1. What is a scientific theory?
  2. Compare and contrast how the term theory is used in science and in everyday language.
  3. Identify two physical science theories.
  4. Relate scientific theories to the law of parsimony.

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