Science operates by a set of core rules that separate it from opinion, philosophy, and guesswork. These rules govern everything from how a single experiment is designed to how findings get shared with the world. Some are about method, some about logic, and some about ethics. Together, they form a framework that makes scientific knowledge uniquely reliable and self-correcting.
Start With a Question, Then Test It
The most fundamental rule of science is that claims must be tested through observation and experiment. This process, known as the scientific method, follows a general sequence: observe something interesting, form a question or hypothesis about it, design an experiment to test that hypothesis, collect data, analyze the results, and draw conclusions. A hypothesis is simply a testable statement about how something works. If you can’t design an experiment that could potentially prove it wrong, it isn’t a scientific hypothesis at all.
Importantly, the scientific method isn’t a rigid recipe. Scientists move back and forth between steps, revise their hypotheses, and redesign experiments based on what they find. But the underlying rule holds firm: every claim must be grounded in evidence gathered through systematic observation or experimentation, not in intuition, authority, or tradition.
Claims Must Be Falsifiable
One of the most important rules in science comes from philosopher Karl Popper: a claim only counts as scientific if it can, in principle, be proven wrong. This is called falsifiability. A scientific theory makes specific predictions about the natural world and invites experiments that could contradict those predictions. Einstein’s theory of relativity, for example, made precise claims that could be tested and potentially disproven through observation. That’s what made it scientific.
By contrast, a claim that explains everything no matter what happens isn’t scientific. Popper pointed to Freud’s psychoanalytic theory as an example. For any given patient, it made no specific predictions that an experiment could contradict. Since nothing could disprove it, Popper considered it unfalsifiable and therefore outside the boundaries of science. This rule acts as a gatekeeper, drawing a clear line between science and speculation.
Evidence Must Be Empirical
Science demands that evidence come from the physical world, through observation and measurement. This principle, called empiricism, means that personal feelings, logical arguments alone, or appeals to authority don’t count as scientific evidence. You need data you can see, measure, and record.
But our senses are limited and easily fooled, which is why controlled experiments are so important. A well-designed experiment includes controls, which are baseline conditions that help scientists separate the real effect of what they’re testing from background noise. Without controls, there’s no way to know if the results reflect something genuine or just random variation. Controls also account for errors in equipment and measurement, acting as reference points that keep observations objective rather than subjective.
Minimize Bias at Every Step
Human beings are prone to seeing what they expect to see. Science has built specific rules to counteract this. One of the most powerful is blinding. In a single-blind study, participants don’t know whether they’re receiving the real treatment or a placebo. In a double-blind study, neither the participants nor the researchers know who is getting what. This prevents researchers from unconsciously treating groups differently and stops participants from reporting results based on their expectations rather than their actual experience. Double-blinding minimizes confirmation bias, observer bias, and inflated placebo effects.
Beyond blinding, science also uses statistical thresholds to guard against false conclusions. The most common standard is a p-value of 0.05, meaning there’s less than a 5% chance the results occurred by random luck alone. This threshold is admittedly arbitrary, and some researchers have argued it should be lowered to 0.005 for more confidence. But the broader rule is clear: scientists must quantify how likely it is that their results are meaningful rather than simply declaring them so.
Results Must Be Reproducible
A single experiment proving something once isn’t enough. One of science’s most critical rules is that results must hold up when repeated. This takes two forms. Reproducibility means that another scientist, using the same data and methods, should get the same results. Replicability means that an entirely new study, collecting fresh data to answer the same question, should reach consistent conclusions.
For this to work, scientists are expected to share detailed information about their methods, data, and even the computational tools they used. The goal is transparency: anyone with the right expertise should be able to follow the same steps and check the work. When results can’t be reproduced, it signals a potential problem with the original findings, whether from error, bias, or fraud.
Prefer the Simplest Explanation
When two explanations fit the same evidence equally well, science favors the simpler one. This rule is known as Occam’s razor, or the principle of parsimony. It doesn’t mean the simplest explanation is always correct, but it serves as a practical guide: don’t multiply assumptions beyond what the evidence requires. If a straightforward explanation accounts for the data, there’s no reason to invoke a complicated one. This principle shapes how scientists build theories, interpret data, and choose between competing models across virtually every field.
Theories and Laws Play Different Roles
A common misconception is that scientific theories are just unproven guesses waiting to become laws. In reality, theories and laws do entirely different jobs. A law describes what nature does under specific conditions. It predicts outcomes, often in mathematical form. Gravity pulls objects together at a rate you can calculate precisely. A theory, on the other hand, explains how and why nature works the way it does. Evolution by natural selection explains the diversity of life. The germ theory of disease explains why infections spread.
There is no hierarchy between them. A theory doesn’t “graduate” into a law. Both are well-supported by evidence, and both are essential. Laws tell you what will happen; theories tell you why it happens. Scientists consider a mature, well-tested theory to be one of the strongest forms of scientific knowledge, not a lesser version of a law.
Peer Review Before Publication
Before scientific findings reach the public, they typically pass through peer review. This means other experts in the same field independently evaluate the research for sound methods, valid conclusions, and proper analysis. Editors at scientific journals coordinate this process and can reach several decisions: accept the paper outright, request minor revisions, require major revisions with further review, or reject it. If major revisions are needed, the revised paper may go back to the original reviewers, a new set of reviewers, or both.
Peer review isn’t perfect. It can be slow, and reviewers can have their own biases. But it serves as a critical filter, catching errors and weak reasoning before they enter the scientific record. It’s one of the rules that makes science a collective, self-correcting enterprise rather than a collection of individual opinions.
Ethical Rules Protect People
Science involving human participants follows strict ethical rules, most of which trace back to the Belmont Report, a foundational document published by the U.S. Department of Health and Human Services. It established three core principles.
- Respect for persons: People must participate voluntarily and with full knowledge of what the research involves. This is the basis of informed consent, which requires that participants receive clear information about the study’s purpose, procedures, risks, and benefits, that they genuinely understand this information, and that their agreement is given freely without coercion.
- Beneficence: Researchers must minimize potential harms and maximize potential benefits. The guiding rules are simple: do not harm, and ensure the expected benefits justify the risks.
- Justice: The benefits and burdens of research must be distributed fairly. No group should bear a disproportionate share of research risks while another group reaps the benefits.
These ethical rules carry legal weight. Research institutions have review boards that must approve studies before they begin, and violations can result in retracted publications, lost funding, and professional consequences. The rules exist because science’s history includes serious abuses, and the framework ensures that the pursuit of knowledge never comes at the cost of human dignity.