(I will be away on travel this week so will be reposting an old series, edited and updated, that discusses the nature of science and the difference between science and religion. New posts start again on Monday, April 21, 2008.)

Because of my science training and my interest in its history and philosophy I am sometimes called upon to answer the question “what is science?” Most people think that the answer should be fairly straightforward. After all science is such an important and integral part of our lives that everyone feels that they already know what it is and think that the problem of defining science is purely one of finding the right combination of words that captures their intuitive sense.

But as I said in an earlier previous posting, strictly defining something means having demarcation criteria for it, which involves developing a set of necessary and sufficient conditions, and this is extremely hard to do even for seemingly simple things like (say) defining what a dog is. So it should not be surprising that it may be harder to do for an abstract idea like science.

But just as a small child is able, based on its experience with pets, to distinguish between a dog and a cat without any need for formal demarcation criteria, so can scientists intuitively sense what is science and what is not science, based on the practice of their profession, without any need of a formal definition. So scientists do not, in the normal course of their work, pay much attention to whether they have a formal definition of science. If forced to define science (say for the purpose of writing textbooks) they tend to make up some kind of definition that sort of fits with their experience, but such ad-hoc formulations lack the kind of formal rigor that is strictly required of a philosophically sound demarcation criterion.

The absence of an agreed-upon formal definition of science has not hindered science from progressing rapidly and efficiently. Science marches on, blithely unconcerned about its lack of self-definition. People start worrying about definitions of science mainly in the context of political battles, such as those involving so-called intelligent design creationism (or IDC), because advocates of IDC have been using this lack of a formal definition to try to define science in a self-serving way so that their pet idea can be included as science, and thus taught in schools as part of the science curriculum and as an alternative to evolution.

Having a clear-cut demarcation criterion that defines science and is accepted by all would settle this question of whether IDC is science once and for all. But finding a satisfactory demarcation criterion for science has proven to be remarkably difficult.

To set about trying to find criteria that distinguishes between one class of ideas from another class, we do what we usually do in all such cases, we first set about finding all the unambiguous members of each class and see if we can extract common properties of each class.

In the case of science, we look at all the knowledge that is commonly accepted as science by everyone, and see if we can identify what is common among these areas. For example, I think everyone would agree that the subjects that come under the headings of astronomy, geology, physics, chemistry, and biology, and which are studied by university departments in reputable universities, all come under the heading of science. So any definition of science that excluded any of these areas would be clearly inadequate, just as any definition of ‘dog’ that excluded a commonly accepted breed would be dismissed as inadequate.

This kind of exercise is exactly we do when trying to define other things, like art (say). Any definition of art that excluded paintings hanging in reputable museums would be considered an inadequate definition.

Similarly, there is a general consensus that astrology, fortune-telling, and the like are not science. Any definition of science that resulted in those topics being considered science would be considered inadequate.

When we look at the history of the topics studied by people in those named disciplines that are commonly accepted as science, the first thing that we notice is that for a theory to be considered scientific it does not have to be true. Newtonian physics is commonly accepted to be scientific, although it is not considered to be universally true anymore. The phlogiston theory of combustion is considered to be scientific though it has long since been overthrown by the oxygen theory. And so on. In fact, since all knowledge is considered to be fallible and liable to change, truth is, in some sense, irrelevant to the question of whether something is scientific or not, because absolute truth cannot be established.

(A caveat: Not all scientists will agree with me on this last point. Some scientists feel that once a theory is shown to be incorrect, it ceases to be part of science, although it remains a part of science history. Some physicists also feel that many of the current theories of (say) sub-atomic particles are unlikely to be ever overthrown and are thus true in some absolute sense. I am not convinced of this. The history of science teaches us that even theories that were considered rock-solid and lasted millennia (such as the geocentric universe) eventually were overthrown.)

But there is a clear pattern that emerges about scientific theories. All the theories that are or have been considered to be science are (1) naturalistic and (2) predictive.

By naturalistic I mean methodological naturalism and not philosophical naturalism. The latter, I argued in an earlier posting where these terms were defined, is irrelevant to science.

By predictive, I mean that all theories that are considered part of science have the quality of having some explicit mechanism or structure that enable the users of these theories to make predictions, of being able to say what one should see if one did some experiment or looked in some place under certain conditions.

Note that these two conditions are just necessary conditions and by themselves are not sufficient. (See this earlier posting for what those conditions mean.) As such they can only classify theories into “may be science” (if it meets both conditions) or “not science” (if it does not meet either or both conditions.) As such, these two conditions by themselves do not make up a satisfactory demarcation criterion. For example, the theory that if a football quarterback throws a lot of interceptions his team is likely to lose, meets both naturalistic and predictive conditions, but such theories are not usually considered part of science.

But even though we do not have rigorous demarcation criteria for science, the existence of just necessary conditions still has important implications, which I shall explore in later postings.