Theory vs. Hypothesis... And Other Related Terms
In discussions about scientific concepts and research, people often use terms like “theory” and “hypothesis” interchangeably, or they use the terms incorrectly. But those terms each have their own meaning, so let's look at each one and some other words related to them to understand what each one means.
A hypothesis is simply a proposed explanation for some phenomenon or observation. It has little if any hard evidence to back it up, but it does intuitively make sense based on previous knowledge of the subject. A hypothesis is sort of like an educated guess. It is a claim that seems like it would make sense based on what you already know, but it still needs hard evidence to support it.
Once a hypothesis has been developed, the next step is to come up with some type of experiment to test it. Therein lies the importance of creating a good hypothesis. Often, people make an observation about the world and immediately jump to conclusions based on their own anecdotal evidence. But by developing a hypothesis, you get a clear and precise statement that can be tested to determine its validity.
There is perhaps no term in science that is more often misused and abused than the word “theory”, and this is largely due to the fact that the word has a very different meaning to scientists than it does to everyone else. To most people, a theory is something that is unproven or lacks evidence. But to scientists, a theory is exactly the opposite of that. In other words, a scientific theory is a claim that has rigorous evidence to support it.
As an example, consider evolution. Opponents of evolution state that it is “only” a theory, and is thus unreliable. But in science, a theory is a claim that has substantial evidence behind it. In the case of evolution, that evidence is everywhere. In fact, the evidence in favor of evolution can be found literally everywhere in biology, from the smallest molecular biology to the behavior of entire ecosystems. When people try to discredit evolution by calling it “only” a theory, they are misusing the word “theory”. When scientists call evolution a theory, they are stating that it is a well observed and strongly supported claim with mountains of evidence behind it.
A postulate, also called an axiom, is a term used in math to refer to a claim that is agreed to be true and proven. Mathematicians would never accomplish anything if they had to re-prove everything before showing new breakthroughs. Instead, they simply state those existing proofs as postulates, which says, “we all agree that this is true. Based on that, here is my new work...”. The term “postulate” is mostly only used in math and philosophy.
A theorem is a conclusion that has been proven based on given postulates. Because postulates are agreed to be true, a mathematical proof based on them can also be considered true. And, because a theorem is proven, it can be used as grounds for other theorems. As with the term “postulate”, the word “theorem” is typically only used in math.
A conjecture is a claim that is believed to be true, but has not been proven or disproved. This is different than a hypothesis. A hypothesis states: “This might be true, or it might not. We don't know.” A conjecture says, “We think this is true, but we can't prove it yet.” A hypothesis does not make assumptions about the truth or falsehood of a claim, it simply states a possible explanation that needs to be tested. A conjecture is believed to be true, but remains open to the possibility that it might be wrong. The term “conjecture” is mostly only used in math.
In science, a law is a statement that is backed by substantial evidence and will always hold true given the conditions described in the law. That second part is very subtle but important. A law only applies when given a specific set of conditions, and may or may not hold up when those conditions are not provided.
As an example, consider what is perhaps the most popular scientific law; Newton's law of universal gravitation. I'm summarizing it here, but the law basically states that every object in the universe exerts a gravitational pull on every other object in the universe, and the gravitational force between them is dependent on the mass of the objects and the distance between them. This law is backed by experimental evidence. However, in unusual conditions, there are situations where the law does not hold. For instance, the law starts to break down when the objects are moving close to the speed of light or if either of the objects is a super massive object like a black hole. In those unusual cases, Newton's law of universal gravitation cannot be assumed to be true, because the law assumes certain conditions that are not met in those situations.
A law is different than a theory. Both are backed by strong evidence, but a law only describes some phenomenon without providing an explanation for it. For example, Newton's law describes the force of gravity, but it does not attempt to explain how gravity actually works.
In contrast, a theory does provide an explanation of the phenomenon in question. For instance, evolution is not a law, it is a theory, because in addition to saying that species arise from other species, it also gives an explanation of how that happens. In effect, it explains evolution as the result of natural selection.
The definitions given here are those used by scientists to explain their work. But keep in mind that the terminology is often misused and misunderstood in normal conversations, so always be sure to take that into account and clarify the context in which they are being used.
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