Scientific Laws and Theories

I've had a student ask me to clarify the difference between a scientific "law" and a "theory". This person asked, in part:

 ".. Is a law, in essence, something which has no detractors --> a unifying 'concept' for which scientists (at the present time) are in accordance with? Is a law a single idea by which all scientists, regardless of discipline, conform?"

 " Can a theory be looked at as a 'transitory' law (i.e., a law in waiting)? In contrast to a law, is it correct to say that there can be several scientific theories about a particular phenomena whereas a law represents a single unified agreement among all scientists".

Such questions are very common. The difference between a "law" and a "theory" often confuses people. This happens, in part, because even among scientists there can be different usage of these terms. Of course, to the general public, these terms have very different meanings and connotations. I suggest that you look up the definitions of both words in any English dictionary.

As used in science, I think that it is important to realize that, in spite of the differences (see below), these terms share some things in common. Both are based on tested hypotheses; both are supported by a large body of empirical data; both help unify a particular field;both are widely accepted by the vast majority (if not all) scientists within a discipline. Furthermore, both scientific laws and scientific theories could be shown to be wrong at some time if there are data to suggest so.

 Presumably the acceptance of laws/theories also applies across disciplines, although most "Laws" or "Theories" are discipline specific. I can't think of law or theory that really transcends all disciplines per se; there is, as of yet, no "Unified Law (or Theory) of Everything." Most scientists aren't trained to critically analyze the pros and cons of laws or theories outside our field. For example, biologists usually aren't qualified (by training) to critique the "Theory of Relativity" or "The Atomic Theory". I don't think a physicist, chemist or engineer (by training) is qualified to discuss the details of the "Theory of Evolution" or the "Cell Theory" either.

  As far as "detractors", the nature of science is to question things, nothing is (or should be) sacrosanct. But, this does not necessarily mean that just because someone questions a law (or theory) that the law/theory in question is wrong. Was Einstein a detractor of Newton when he showed that the Newtonian "Laws" of mechanics did not explain everything (wasn't that why quantum mechanics came into existence)? Just because Newtonian mechanics is "wrong" in some situations, does that mean it is useless? I don't think so!! If certain aspects of evolutionary theory (e.g., natural selection, gradualism) has "detractors" (and I mean among people who are qualified to argue about it -- among biologists), does that mean natural selection (or the idea of biological evolution in general) is wrong? NO!! Scientific knowledge is strengthened by people questioning what is or has been accepted.

Here are a couple of definitions of each word.


1) An empirical generalization; a statement of a biological principle that appears to be without exception at the time it is made, and has become consolidated by repeated successful testing; rule (Lincoln et al., 1990)

 2) A theoretical principle deduced from particular facts, applicable to a defined group or class of phenomena, and expressible by a statement that a particular phenomenon always occurs if certain conditions be present (Oxford English Dictionary as quoted in Futuyma, 1979).

 3) A set of observed regularities expressed in a concise verbal or mathematical statement. (Krimsley, 1995).


 1) The grandest synthesis of a large and important body of information about some related group of natural phenomena (Moore, 1984)

 2) A body of knowledge and explanatory concepts that seek to increase our understanding ("explain") a major phenomenon of nature (Moore, 1984).

 3) A scientifically accepted general principle supported by a substantial body of evidence offered to provide an explanation of observed facts and as a basis for future discussion or investigation (Lincoln et al., 1990).

 4) 1. The abstract principles of a science as distinguished from basic or applied science. 2. A reasonable explanation or assumption advanced to explain a natural phenomenon but lacking confirming proof (Steen, 1971). [NB: I don't like this one but I include it to show you that even in "Science dictionaries" there is variation in definitions which leads to confusion].

 5) A scheme or system of ideas or statements held as an explanation or account of a group of facts or phenomena; a hypothesis that has been confirmed or established by observation or experiment, and is propounded or accepted as accounting for the known facts; a statement of what are held to be the general laws, principles or causes of something known or observed. (Oxford English Dictionary, 1961; [emphasis added]).

 6) An explanation for an observation or series of observations that is substantiated by a considerable body of evidence (Krimsley, 1995).

Given my above arguments for how similar these two words are, it is nonetheless true that "law" and "theory" are different words that can or do have different connotations. So, what's the difference? Look above at the last definitions under Law and Theory. These definitions clearly differentiate the two words. Some scientists will tell you that the difference between them is that a law describes what nature does under certain conditions, and will predict what will happen as long as those conditions are met. A theory explains how nature works. Others delineate law and theory based on mathematics -- Laws are often times mathematically defined (once again, a description of how nature behaves) whereas theories are often non-mathematical. Looking at things this was helps to explain, in part, why physics and chemistry have lots of "laws" whereas biology has few laws (and more theories). In biology, it is very difficult to describe all the complexities of life with "simple" (relatively speaking!) mathematical terms.  

Regardless of which definitions one uses to distinguish between a law and a theory, scientists would agree that a theory is NOT a "transitory law, a law in waiting". There is NO hierarchy being implied by scientists who use these words. That is, a law is neither "better than" nor "above" a theory. From this view, laws and theories "do" different things and have different roles to play in science. Furthermore, notice that with any of the above definitions of law, neither scientists nor nature "conform" to the law. In science, a law is not something that is dictated to scientists or nature; it is not something that a scientist or nature has to do under threat of some penalty if they don't conform.

Literature Cited

Futuyma, D. J. 1979. Evolutionary Biology. Sinauer Assoc.

Krimsley, V. S. 1995. Introductory Chemistry, 2nd Ed. Brooks/Cole Publishing Co., Pacific Grove.

Lincoln, R. J., G. A. Boxshall, and P. F. Clark. 1990. A dictionary of ecology, evolution and systematics. Cambridge Univ. Press.

Moore, J. A. 1984. Science as a way of knowing--evolutionary biology. Amer. Zool. 24: 467-534.

Oxford English Dictionary, 1961; Oxford University Press, London.

Steen, E. B. 1971. Dictionary of Biology. Barnes and Nobel.