When ancient Greeks sprained their ankles, caught the flu, or accidentally set their togas on fire, they had to choose between two kinds of doctors: dogmatists (from dogmatikos, meaning “belief”), who thought that the best way to understand illness was to develop theories about the body’s functions, and empiricists (from empeirikos, meaning “experience”), who thought that the best way to understand illness was to observe sick people. The rivalry between these two schools of medicine didn’t last long because the people who went to see dogmatists tended to die, which was bad for business. Today we use the word dogmatism to describe the tendency for people to cling to their assumptions, and the word empiricism to describe the belief that accurate knowledge can be acquired through observation. The fact that we can answer questions about the natural world by examining it may seem painfully obvious to you, but for most of human history, people mainly trusted authorities to answer important questions, and it is only in the last millennium (and especially in the past three centuries) that people have begun to trust their eyes and ears more than their elders.

Empiricism is the essential element of the scientific method, which is a procedure for finding truth by using empirical evidence. In essence, the scientific method suggests that when we have an idea about the world—about how bats navigate, or about why people can’t forget traumatic events—we should gather empirical evidence relevant to that idea and then, if necessary, modify the idea to fit with the evidence. Scientists usually refer to an idea of this kind as a theory, which is a hypothetical explanation of a natural phenomenon. For example, we might theorize that bats navigate by making sounds and then listening for the echo, or that traumatic events cause the body to release chemicals that burn memories into the brain.

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How do we decide if a theory is right? Theories make specific predictions about what we should observe. For example, if bats really do navigate by making sounds and then listening for echoes, then we should observe that deaf bats can’t navigate. That “should” statement is technically known as a hypothesis, which is a falsifiable prediction made by a theory. The word falsifiable is a critical part of that definition. Some theories, such as “God created the universe,” simply do not specify what we should observe if they are or are not true, and thus no observation can ever falsify them. Because these theories do not give rise to hypotheses, they can never be the subject of scientific investigation. That doesn’t mean they’re wrong—it just means that we can’t evaluate them by using the scientific method.

So what happens when we test a hypothesis? Albert Einstein is reputed to have said: “No amount of experimentation can ever prove me right, but a single experiment can prove me wrong.” Why should that be? Well, just imagine what you could learn about the navigation-by-sound theory by observing a few bats. If you saw the deaf bats navigating every bit as well as the hearing bats, then the navigation-by-sound theory would instantly be proved wrong; on the other hand, if you saw the deaf bats navigating more poorly than the hearing bats, your observation would be consistent with the navigation-by-sound theory but would not prove it. After all, even if you didn’t see a deaf bat navigating perfectly today, it is still possible that you will see one tomorrow. When evidence is consistent with a theory, it increases our confidence in that theory, but it never makes us completely certain. The next time you see a news headline that says, “Scientists prove theory X correct,” you are hereby authorized to roll your eyes.

The scientific method suggests that the best way to learn the truth about the world is to develop theories, derive hypotheses from them, test those hypotheses by gathering evidence, and then use that evidence to modify the theories when necessary. Gathering evidence properly requires an empirical method, which is a set of rules and techniques for observation. Because human behavior is fairly easy to observe, you might expect psychology’s empirical methods to be fairly simple. In fact, the methodological challenges facing psychologists are among the most daunting in all of modern science because three things make humans especially difficult to study:

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The fact that human beings are complex, variable, and reactive presents a major challenge to the scientific study of their behavior, but two kinds of methods allow us to meet these challenges head-on: methods of observation which allow us to determine what people do, and methods of explanation which allow us to determine why people do it. We’ll examine both kinds of methods in the sections that follow.