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1.4: Scientific Theories

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    What causes disease?

    Today most people realize that microorganisms, such as bacteria or viruses, are the cause of disease. This concept is known as the germ theory of disease, one of the few scientific theories in the field of the life sciences. Although it seems obvious now, people did not always understand the cause of disease. How does a theory such as this become established?

    Scientific Evidence and Theories

    One goal of a scientist is to find answers to scientific questions. To do this, scientists first develop a hypothesis, which is a proposed explanation that tries to explain an observation. To collect evidence to support (or disprove) their hypothesis, scientists must do experiments. Evidence is:

    1. A direct, physical observation of something or a process over time.
    2. Usually something measurable or "quantifiable."
    3. The data resulting from an experiment.

    For example, an apple falling to the ground is evidence in support of the law of gravity. A bear skeleton in the woods would be evidence of the presence of bears.

    Looking at the image below might be confusing at first because this evidence seems to defy the law of gravity (Figure below). Of course water cannot be poured out of bottle and flow upward. The law of gravity is a scientific law, which is a statement describing what always happens under certain conditions in nature. Scientific laws are developed from lots of collected information.

    Water going upwards, defying the law of gravity
    Figure \(\PageIndex{1}\): Water going upward against gravity.

    If many experiments are performed, and lots of evidence is collected in support of a general hypothesis, a scientific theory can be developed. Scientific theories are well established explanations of evidence, usually tested and confirmed by many different people. Scientific theories usually have a lot of evidence in support of the theory, and no evidence disproving the theory. Scientific theories produce information that helps us understand our world. For example, the idea that matter is made up of atoms is a scientific theory. Scientists accept this theory as a fundamental principle of basic science.

    A scientific theory must stand up to all scientific testing. Thus, when scientists find new evidence, they can change their theories. In addition to the germ theory of disease, other scientific theories are the cell theory and the theory of evolution.

    Further Reading



    • Evidence is a direct, physical observation of something or a process.
    • Scientific theories are explanations of some aspect of the natural world based on repeated observations.

    Explore More

    Use the resource below to answer the questions that follow.

    1. What happens to scientific ideas that do not match the natural world?
    2. In science, what is meant by a fact, a hypothesis, a theory, and a law?
    3. How do scientists' views of theories differ from the everyday use of these words?


    1. What is evidence?
    2. What is a scientific theory?
    3. What is a scientific law?

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