Week 1 · Chapter

Chapter 1 — The Science of Psychology

Introduction to Psychology Free sample edition

The module's primary reading · ~30–40 minutes · Objective 1 (explain what makes psychology a science and compare its major theoretical perspectives (biological, psychodynamic, behavioral, cognitive, humanistic, sociocultural)) · Read it first this week; the lecture, the tutorial, and the quiz all build on it.

In plain language

Psychology is the science of what people do and what goes on inside their heads. This week you will learn what the field studies, where it came from, and the six main ways psychologists explain the same behavior. You will see why the field tests its ideas with evidence instead of trusting common sense (the everyday beliefs about people that feel true without being checked). You will also learn to tell a theory (a broad explanation already backed by evidence) from a hypothesis (a specific prediction that data could prove wrong). Those two habits, asking for evidence and asking for a testable prediction, are what make psychology a science.

Why you already care about this

In 1879, in Leipzig, Germany, Wilhelm Wundt opened a laboratory set aside for one new kind of work: measuring the mind. A signal appeared, a person responded, and an instrument timed the gap. In trials like the ones in this chapter, that gap takes about a third of a second. It sounds like a small thing to measure. It was the start of a science.

You already ask the questions that science grew to answer. Why did your friend go quiet in the group chat? Why do you crave chips at 11 p.m. when you were not hungry at dinner? Why did a stranger on the bus hand back the glove you dropped? You probably have an answer ready for each one. So does everyone else, and the answers often disagree.

Psychology's job is to find out which answers hold up. This chapter shows you how the field began, the six lenses it uses, and the two habits that separate a scientific claim from a confident guess.

By the end of this chapter you will be able to:

tell a behavior (something you can observe) from a mental process (something you infer);
summarize a small set of reaction times by its mean, its median, and its range, and say which one better describes a typical trial;
name the founder and the central question of each major school in the field's history;
explain one behavior from all six perspectives and combine them with the biopsychosocial model;
spot hindsight bias in a "that's obvious" reaction to a finding;
sort a statement into a theory or a hypothesis, and say whether data could prove it wrong.

Carry this line with you: a feeling of certainty is not evidence; a test is.

Where we are

This week: what psychology studies, its history in six moves, the six perspectives, and why the field relies on evidence rather than common sense.
How they connect: this chapter is the foundation, so there is no earlier week to lean on; every later week uses the lenses and habits you build here.
Where we're going next: Week 2, Research Methods & Ethics, turns the habit of asking for evidence into the tools psychologists use to gather it.

This is the first week of the course, so nothing earlier is assumed. You need no background in science and no math beyond adding and dividing. What you do need is curiosity about people, including yourself. Next week you will learn the research designs that turn a hypothesis into data, so this week's last section sets that up.

1. Psychology defined: study what people do and what they think

Picture a classmate in the front row. You can see her tap her pen, check the clock, and pack her bag five minutes early. You cannot see her worry about the shift she has to make it to after class. You can only guess at it from what she does.

That split, between what you can see and what you have to infer, is built into the definition of the field.

Psychology is the scientific study of behavior and mental processes: what people and animals observably do, and what can be inferred about how they think and feel, tested against systematically gathered evidence.

The two halves of that definition each have a name.

Behavior is anything a person or animal does that someone else could observe and record, such as speaking, pressing a key, or walking away.
Mental processes are the inner experiences that cannot be watched directly, such as thinking, remembering, feeling, and perceiving, which psychologists infer from behavior.

Notice the word scientific. Psychology is not the only field that cares about people. Novelists, coaches, and grandparents all know a great deal about human nature. What makes psychology different is its method: it checks its claims against evidence gathered in a careful, repeatable way.

Worked example — the reaction-time trial. Here is the simplest experiment from the field's first laboratory. You sit in front of a lamp with one finger on a key. When the lamp flashes, you press the key as fast as you can.

What is the behavior? Your finger pressing the key. Anyone in the room could see it, and a clock can record exactly when it happened.
What is the mental process? Noticing the flash and deciding to move. Nobody can see that happen inside you.
How does the behavior reveal the process? The gap between the flash and the press is the reaction time: the time, measured in milliseconds, between a signal such as a flash of light and a person's response to it, such as a key press. A longer gap suggests more mental work happened between seeing and doing.

In words: the key press is the behavior you record, and the reaction time is the window it gives you onto a mental process you cannot see.

Misconception check. Students often think psychology is "only about the mind," so a behavior like a key press seems too small to matter. The idea is tempting because the mind is the interesting part. But the mind cannot be measured directly. Psychology studies mental processes through behavior, so a careful record of what people do is how the field gets at what they think.

Memory hook: behavior is what you see; a mental process is what you infer.

2. The first laboratory: measure a mental process in milliseconds

Go back to that room in Leipzig. Wilhelm Wundt opened his laboratory for experimental psychology there in 1879, and that year is the usual birth date given for psychology as an experimental science. Two kinds of work went on in that room.

The first was introspection: the method of Wundt's laboratory in which trained observers looked inward and reported the parts of their own conscious experience under controlled conditions. An observer might listen to a single tone and describe each sensation it produced. The second kind of work used instruments that timed reactions, so a mental process could leave a number behind.

The numbers are what lasted. To see what such a measurement looks like, take the reaction-time sample below. It is an illustrative sample constructed for this course, not real data. Each value is one simple-reaction-time trial: a light flashes and a person presses a key. One millisecond (ms) is a thousandth of a second, so 1 second is 1,000 ms, and a 312 ms trial takes about a third of a second.

Nine simple-reaction-time trials, in milliseconds (illustrative, constructed for this course, not real data)
Trial	1	2	3	4	5	6	7	8	9
Time (ms)	312	289	401	356	298	275	388	342	310

Nine numbers are hard to hold in your head. So you summarize them. Three summaries do most of the work.

The mean is the sum of a set of values divided by how many values there are; it is pulled toward a few unusually large or small values. Its symbol is x̄.
The median is the middle value of a set once the values are sorted from smallest to largest; a few unusually large or small values barely move it.
The range is the largest value minus the smallest value in a set, which tells how far the values stretch from one end to the other.

We write n for the number of trials measured.

Worked example — summarizing the reaction-time sample. Work through the five steps in order.

Count. There are n = 9 trials.
Total. Add every trial: 312 + 289 + 401 + 356 + 298 + 275 + 388 + 342 + 310 = 2,971 ms.
Mean. Divide the total by the count: x̄ = 2,971 ⁄ 9 = 330.1111 ms.
Median. Sort the trials: 275, 289, 298, 310, 312, 342, 356, 388, 401. With 9 values, the middle one is the 5th, so the median is 312 ms.
Range. Take the largest minus the smallest: max − min = 401 − 275 = 126 ms.

Now read the numbers. The mean (330.1111 ms) sits above the median (312 ms). Why? A few slow trials, 388 ms and 401 ms, pull the mean up. The slowest trial took 401 versus 275 ms for the fastest, a ratio of about 1.5 to 1. The median ignores how slow the slowest trials were, so it is the better picture of a typical trial in this sample.

In words: a typical trial here took a little over 300 ms, about a third of a second, and a few slow trials stretch the set out to a range of 126 ms.

Misconception check. Students often think the mean is always the "typical" value, because it is the summary they learned first. That is tempting, since the mean uses every number. But one or two unusual trials can drag the mean away from where most values sit. When a few values run long, as they do here, the median describes the typical trial better.

Honesty note: introspection did not last as a scientific method. Two trained observers could report the same sound in different words, and no one could check who was right. Instruments that timed behavior gave everyone the same number to argue about. That is a lesson the field kept.

Memory hook: Wundt timed the gap; the gap became the data.

3. History in six moves: trace who asked which question

Imagine six people walk into the same room and ask what the mind is. One asks what it is made of. One asks what it is for. One asks what it hides. One says to ignore it and watch what people do. One asks what people could become. One asks how it handles information. That room is a short history of psychology. Here are the six moves, in order.

Move 1 — Wundt's laboratory (1879). You met it in Section 2: introspection plus timed reactions, in Leipzig.

Move 2 — structuralism versus functionalism. Edward Titchener trained under Wundt and carried the work forward.

Structuralism is an early school of psychology, built by Edward Titchener, that tried to break conscious experience into its basic elements through introspection.
Functionalism is an early school of psychology, led by William James, that asked what mental processes are for: how they help an organism adapt to its surroundings.

Think of a bicycle. A structuralist lists its parts: the chain, the gears, the brakes. A functionalist asks what the bicycle does for its rider and why it is shaped that way.

Move 3 — Freud and psychoanalysis. Sigmund Freud (1856–1939) built a very different tradition.

Psychoanalysis is Sigmund Freud's approach, which holds that wishes and conflicts outside a person's awareness shape behavior, and which treats people by bringing those conflicts into awareness.

Move 4 — behaviorism. In a 1913 paper, John B. Watson argued that psychology should stop guessing at the mind and study only what can be observed. B. F. Skinner later extended the idea.

Behaviorism is the school, led by John B. Watson and later B. F. Skinner, that limited psychology to observable behavior and the consequences that shape it.

Move 5 — the humanistic movement. Abraham Maslow and Carl Rogers pushed back on both Freud and the behaviorists. People, they said, are not only driven by hidden conflicts or by rewards.

The humanistic movement is the movement led by Abraham Maslow and Carl Rogers that centered psychology on personal growth, free choice, and each person's own point of view.

Move 6 — the cognitive and biological eras. Ulric Neisser's 1967 book Cognitive Psychology named and helped launch a return to studying the mind: how people notice, remember, and solve problems. At the same time, better tools for studying the brain and body drew more researchers to ask how biology shapes behavior.

Worked example — matching a question to its school. Read each question and name the school that asked it.

"What basic sensations make up the experience of hearing a bell?" This breaks experience into elements, so it is structuralism (Titchener).
"What does memory do to help a person survive and adapt?" This asks what a process is for, so it is functionalism (William James).
"What reward has followed this habit in the past?" This looks only at behavior and its consequences, so it is behaviorism (Watson, then Skinner).
"What unspoken conflict might this slip of the tongue reveal?" This looks for wishes outside awareness, so it is psychoanalysis (Freud).

In words: each school is defined by the question it asked, and each question still has a home in the field today.

Misconception check. Students, and chatbots, often call Freud the founder of scientific psychology, or credit functionalism to Wundt. Both are tempting because the names are famous and the dates are close. But the pinned history is clear: Wundt's 1879 laboratory marks psychology as an experimental science; Titchener built structuralism; William James led functionalism; Freud founded psychoanalysis, a separate tradition. A misattributed founder is a factual error, not a detail.

Memory hook: name the question each school asked, not only its founder.

4. Six perspectives: explain one behavior six ways

You have a paper due tomorrow morning. It is 10 p.m., and you have written one sentence. You told yourself last week you would start early. Why didn't you?

Six people could give you six different, reasonable answers. Each one is a perspective: one of the main angles from which psychologists explain behavior, each asking its own kind of question about the same event. The schools you met in Section 3 grew into today's perspectives. Here they are, each defined once.

The biological perspective explains behavior through the brain, the body, hormones, and genes.
The psychodynamic perspective, grown from Freud's psychoanalysis, explains behavior through feelings, conflicts, and motives a person may not be aware of.
The behavioral perspective explains behavior through learning: the rewards and consequences that have followed similar behavior in the past.
The cognitive perspective explains behavior through how a person thinks: what the person notices, expects, remembers, and believes.
The humanistic perspective explains behavior through a person's growth, choices, and sense of who they want to become.
The sociocultural perspective explains behavior through other people, groups, and culture: the norms and expectations a person lives among.

Worked example — six lenses on one late paper. A student puts off a paper until the night before. One sentence per perspective:

Biological. Sleep loss and stress hormones may change how the body handles the deadline.
Psychodynamic. The paper may stir an unspoken fear of being judged.
Behavioral. Past last-minute papers that still earned a passing grade rewarded the habit.
Cognitive. The student expects to write faster under pressure and misjudges the time it takes.
Humanistic. The assignment feels disconnected from what the student wants to become.
Sociocultural. Friends who all start late make starting late feel normal.

Notice that none of these cancels the others. They can all be partly true at once. That is the idea behind three levels of analysis: the biological, psychological, and social layers at which any one behavior can be explained, from the body up to the culture around a person.

The biopsychosocial model is the view that biological, psychological, and social influences act together on behavior and health, so a full explanation uses all three levels of analysis. It was proposed by G. L. Engel in 1977. For the late paper, the biological level holds the sleep and the stress; the psychological level holds the fear, the habit, the misjudged time, and the sense of purpose; the social level holds the friends who all start late.

The same six lenses work on any behavior. Take a craving for something salty at 11 p.m. The biological lens asks about hunger signals. The behavioral lens asks what late-night snacks have rewarded before. The sociocultural lens asks whether the people around you snack while watching shows together. Or take a stranger on a bus who hands a dropped glove back to its owner. The humanistic lens asks what kind of person the helper wants to be. The cognitive lens asks what the helper noticed and believed in that moment.

Misconception check. Students often think the six perspectives are rival teams and only one can be right. That is tempting, because each perspective sounds complete on its own. But they work at different levels, and the biopsychosocial model holds them together. The useful question is not "which lens wins?" but "what does each lens add?"

Pause and reflect — choose one. Pick either prompt; they count the same.

(a) Think of a habit of your own you would like to change. Which two perspectives best explain it, and why those two?
(b) Think of a time two people explained the same event in completely different ways. Which perspectives were they each using?

Memory hook: one behavior, six lenses, three levels — not six rival answers.

5. Evidence over common sense: test what feels obvious

Here is a saying you have heard: "Birds of a feather flock together." Here is another: "Opposites attract." Both sound wise. Both feel true when you hear them. But they predict opposite things about who will become friends. Common sense has a saying ready for every outcome, which means it cannot tell you, ahead of time, which outcome to expect.

Psychology answers that problem with one principle.

Empiricism is the principle that claims should be tested against careful observation and gathered evidence rather than accepted because they feel true or sound sensible.

Common sense is not useless. It is a good source of ideas to test. The trouble starts when it is treated as the test itself.

A second trap makes common sense feel stronger than it is. Suppose you hear about a study showing that people work better in teams. "Obvious," you think. Now suppose you heard the reverse, that people work better alone. That might feel obvious too, because you can picture a group project that dragged.

Hindsight bias is the tendency to see an outcome as having been predictable after it is known, the feeling of having known it all along. After a basketball game, fans explain exactly why the winning team was always going to win. Before the tip-off, some of those same fans may have picked the other side.

Worked example — catching hindsight bias. A friend reads a headline that says students who take short breaks while studying remember more. "Everyone knows that," your friend says.

Ask the reverse question. Would "students who study without breaks remember more" also have sounded obvious? It probably would: staying focused sounds like good advice too.
Notice the pattern. If both a finding and its opposite feel obvious, the feeling is not evidence. It is hindsight bias.
Name what would settle it. Only gathered evidence, observing what students actually remember under each way of studying, can tell the two claims apart.

In words: "I knew it all along" is a feeling that shows up after the answer, so it cannot be the reason to believe the answer.

Misconception check. Students often think common sense already explains people, so psychology only puts fancy names on what everyone knows. That is tempting, because many findings do sound familiar once you hear them. But common sense explains everything after the fact and predicts nothing before it. Hindsight bias is why a finding feels obvious once you know it.

Caveat: being skeptical of common sense does not mean being cynical about people's experience. Your own sense of what happened is real data about you. Empiricism asks only that a claim about people in general be checked before it is trusted.

Memory hook: if the opposite would feel obvious too, the feeling is not evidence.

6. Theory versus hypothesis: tell the explanation from the prediction

A food-truck owner notices that sales rise on days the line looks long. She has a broad idea: people trust a place other people already chose. That idea could explain many things, from busy restaurants to popular videos. But on its own it does not tell her what to check tomorrow. For that, she needs a narrower statement.

Those two statements have names, and the difference between them is one of the most important in science.

A theory is a broad explanation that organizes many observations and is already backed by evidence, and that generates new predictions to test.
A hypothesis is a specific prediction, usually drawn from a theory, stated precisely enough that data could show it to be wrong.

That last phrase has its own word. A claim is falsifiable when it is stated so that some possible observation could show it to be wrong; a claim that fits every possible outcome is not falsifiable.

Here is how the two fit together. A theory generates a hypothesis. Data test the hypothesis. If the prediction fails, the theory gets revised or dropped. If it holds, and holds again when others repeat the test, the theory gains support.

Worked example — sorting three statements. Decide what each statement is, and why.

"People are influenced by what other people around them choose." This is broad and organizes many observations, so it is the theory.
"On days when the truck's line has eight or more people at noon, more walk-up customers will join it in the next hour than on days when the line has two or fewer." This names exactly what to count and what to expect, so it is a hypothesis. Data could prove it wrong: if the short-line days draw as many walk-ups, the prediction fails.
"Customers always do whatever feels right to them." No observation could contradict this, because any choice can be called what felt right. It is not falsifiable, so it is neither a useful hypothesis nor a scientific theory.

In words: a theory is the explanation, a hypothesis is the testable prediction, and a statement that no result could contradict is not science yet.

Misconception check. Students often think a theory is "only a guess," the way people say "I have a theory about why the bus is late." That everyday use is tempting, because it is how most people talk. But in science, a theory is the opposite of a guess: it is an explanation already backed by evidence. The specific, testable guess is the hypothesis.

Limitation: no single study proves a theory true for good. A theory is the best explanation the evidence supports so far, which is why scientists keep testing it.

Memory hook: the theory explains; the hypothesis predicts; the data decide.

Key terms

common sense — The everyday beliefs about people that feel true without being checked. (first used: In plain language)
psychology — The scientific study of behavior and mental processes: what people and animals observably do, and what can be inferred about how they think and feel, tested against systematically gathered evidence. (first used: 1. Psychology defined: study what people do and what they think)
behavior — Anything a person or animal does that someone else could observe and record, such as speaking, pressing a key, or walking away. (first used: 1. Psychology defined: study what people do and what they think)
mental processes — The inner experiences that cannot be watched directly, such as thinking, remembering, feeling, and perceiving, which psychologists infer from behavior. (first used: 1. Psychology defined: study what people do and what they think)
reaction time — The time, measured in milliseconds, between a signal such as a flash of light and a person's response to it, such as a key press. (first used: 1. Psychology defined: study what people do and what they think)
introspection — The method of Wundt's laboratory in which trained observers looked inward and reported the parts of their own conscious experience under controlled conditions. (first used: 2. The first laboratory: measure a mental process in milliseconds)
mean — The sum of a set of values divided by how many values there are; it is pulled toward a few unusually large or small values. (first used: 2. The first laboratory: measure a mental process in milliseconds)
median — The middle value of a set once the values are sorted from smallest to largest; a few unusually large or small values barely move it. (first used: 2. The first laboratory: measure a mental process in milliseconds)
range — The largest value minus the smallest value in a set, which tells how far the values stretch from one end to the other. (first used: 2. The first laboratory: measure a mental process in milliseconds)
structuralism — An early school of psychology, built by Edward Titchener, that tried to break conscious experience into its basic elements through introspection. (first used: 3. History in six moves: trace who asked which question)
functionalism — An early school of psychology, led by William James, that asked what mental processes are for: how they help an organism adapt to its surroundings. (first used: 3. History in six moves: trace who asked which question)
psychoanalysis — Sigmund Freud's approach, which holds that wishes and conflicts outside a person's awareness shape behavior, and which treats people by bringing those conflicts into awareness. (first used: 3. History in six moves: trace who asked which question)
behaviorism — The school, led by John B. Watson and later B. F. Skinner, that limited psychology to observable behavior and the consequences that shape it. (first used: 3. History in six moves: trace who asked which question)
humanistic movement — The movement led by Abraham Maslow and Carl Rogers that centered psychology on personal growth, free choice, and each person's own point of view. (first used: 3. History in six moves: trace who asked which question)
perspective — One of the main angles from which psychologists explain behavior, each asking its own kind of question about the same event. (first used: 4. Six perspectives: explain one behavior six ways)
biological perspective — The perspective that explains behavior through the brain, the body, hormones, and genes. (first used: 4. Six perspectives: explain one behavior six ways)
psychodynamic perspective — The perspective, grown from Freud's psychoanalysis, that explains behavior through feelings, conflicts, and motives a person may not be aware of. (first used: 4. Six perspectives: explain one behavior six ways)
behavioral perspective — The perspective that explains behavior through learning: the rewards and consequences that have followed similar behavior in the past. (first used: 4. Six perspectives: explain one behavior six ways)
cognitive perspective — The perspective that explains behavior through how a person thinks: what the person notices, expects, remembers, and believes. (first used: 4. Six perspectives: explain one behavior six ways)
humanistic perspective — The perspective that explains behavior through a person's growth, choices, and sense of who they want to become. (first used: 4. Six perspectives: explain one behavior six ways)
sociocultural perspective — The perspective that explains behavior through other people, groups, and culture: the norms and expectations a person lives among. (first used: 4. Six perspectives: explain one behavior six ways)
levels of analysis — The biological, psychological, and social layers at which any one behavior can be explained, from the body up to the culture around a person. (first used: 4. Six perspectives: explain one behavior six ways)
biopsychosocial model — The view that biological, psychological, and social influences act together on behavior and health, so a full explanation uses all three levels of analysis. (first used: 4. Six perspectives: explain one behavior six ways)
empiricism — The principle that claims should be tested against careful observation and gathered evidence rather than accepted because they feel true or sound sensible. (first used: 5. Evidence over common sense: test what feels obvious)
hindsight bias — The tendency to see an outcome as having been predictable after it is known, the feeling of having known it all along. (first used: 5. Evidence over common sense: test what feels obvious)
theory — A broad explanation that organizes many observations and is already backed by evidence, and that generates new predictions to test. (first used: 6. Theory versus hypothesis: tell the explanation from the prediction)
hypothesis — A specific prediction, usually drawn from a theory, stated precisely enough that data could show it to be wrong. (first used: 6. Theory versus hypothesis: tell the explanation from the prediction)
falsifiable — Describes a claim stated so that some possible observation could show it to be wrong; a claim that fits every possible outcome is not falsifiable. (first used: 6. Theory versus hypothesis: tell the explanation from the prediction)

Summary

Psychology studies behavior, which you can observe, and mental processes, which you infer from behavior.
Wilhelm Wundt's Leipzig laboratory, opened in 1879, marks psychology's start as an experimental science.
A reaction time turns a mental process into a number; the mean, median, and range summarize a set of them.
When a few values run long, the median describes the typical value better than the mean.
Titchener built structuralism; William James led functionalism; Freud founded psychoanalysis; Watson and Skinner led behaviorism.
Maslow and Rogers led the humanistic movement; the cognitive and biological eras followed.
The six perspectives explain one behavior at different levels; the biopsychosocial model combines them.
Empiricism means testing claims with evidence; hindsight bias makes findings feel obvious after the fact.
A theory explains and is backed by evidence; a hypothesis predicts and must be falsifiable.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

A cycling coach times how fast five riders react to a starting signal: 250, 230, 410, 240, and 260 ms. Find the mean, the median, and the range. Which better describes a typical rider, and why?
A shopper picks up a jacket, frowns at the price tag, and puts it back. Later she says she felt it was overpriced. Which parts of this are behavior, and which part is a mental process?
A nurse on a night shift notices that a coworker snaps at people near the end of every long shift. Give one explanation from the biological perspective and one from the sociocultural perspective.
Sort each statement as a theory, a hypothesis, or not falsifiable: (a) "Library visitors who are greeted by name at the desk will borrow more books that month than visitors who are not greeted by name." (b) "Everything people do is secretly driven by self-interest, even when it looks generous." (c) "People repeat actions that have been followed by rewards."

Answers (worked reasoning — read after attempting)

Add the times: 250 + 230 + 410 + 240 + 260 = 1,390 ms. Divide by the count: 1,390 ⁄ 5 = 278 ms, so the mean is 278 ms. Sort them: 230, 240, 250, 260, 410. The middle of five values is the 3rd, so the median is 250 ms. The range is 410 − 230 = 180 ms. The median better describes a typical rider: four of the five riders reacted between 230 and 260 ms, and one slow start at 410 ms pulled the mean up to 278 ms.
Picking up the jacket, frowning, putting it back, and saying it was overpriced are all behaviors: anyone nearby could see or hear them. Her judgment that the jacket was overpriced is a mental process. Nobody can see that judgment directly; you infer it from what she did and said.
Biological: after many hours awake, tiredness and stress hormones may make it harder for the coworker to stay calm. Sociocultural: if snapping at the end of a shift is common and accepted on that unit, the workplace's norms may make it feel normal. Both can be true at once, which is the point of the biopsychosocial model.
(a) is a hypothesis: it names exactly what to measure (books borrowed in a month) and what to expect, and the data could show no difference. (b) is not falsifiable: any generous act can be reinterpreted as secret self-interest, so no observation could prove it wrong. (c) is a theory: it is a broad explanation that organizes many observations and generates specific predictions, such as (a)-style tests in many settings.

Next up in this module: the Readings & Resources page, the lecture tutorial, the practice set, and the quiz — and next week the scientific method turns each hypothesis into a research design you can test.

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