1.10: When do hypotheses become theories, and what are theories?

It’s an unfortunate source of confusion that, among the general public and in the popular media, the word “theory” is often used to refer to a hunch or a guess or speculation—that is, something we are not certain about. In fact, to scientists, the word means nearly the opposite: a hypothesis of which they are most certain. To reduce these common misunderstandings, we examine here the ways in which scientists describe our knowledge about natural phenomena.

Hypothesis As we have seen, hypotheses are at the very heart of scientific thinking. A hypothesis is a proposed explanation for a phenomenon. A good hypothesis leads to testable predictions. Commonly, when non-scientists use the word “theory”—as in, “I’ve got a theory about why there’s less traffic on Friday mornings than on Thursday mornings”—they actually mean that they have a hypothesis.

Theory A theory is an explanatory hypothesis for natural phenomena that is exceptionally well supported by the empirical data. A theory can be thought of as a hypothesis that has withstood the test of time and is unlikely to be altered by any new evidence. Like a hypothesis, a theory is testable; but because it has already been repeatedly tested and no observations or experimental results have contradicted it, a theory is viewed by the scientific community with nearly the same confidence as a fact. For this reason, it is inappropriate to describe something as “just a theory” as a way of asserting that it is not likely to be true.

Theories in science also tend to be broader in scope than hypotheses. In biology, two of the most important theories (which we explore in more detail in Chapters 3 and 8) are the cell theory, that all organisms are composed of cells and all cells come from preexisting cells, and the theory of evolution by natural selection, that species can change over time and all species are related to one another through common ancestry.