13.1 The Values of Science
Scientists seek not merely to know what the facts are but to explain them, and to this end they devise theories.
13.2 Explanations: Scientific and Unscientific
An explanation is a group of statements from which some event (or thing) can logically be inferred, and whose acceptance removes or diminishes the problematic or puzzling character of that event (or thing). The inference created from an explanation makes the conclusion follow not deductively, but rather with probability. Inference and explanation are therefore closely related. Given certain premisses, any conclusion that can logically be inferred from them can be regarded as being explained by them. On the other hand, given a fact to be explained, we can say that we have provided an explanation for it when we have found a set of premisses from which it can logically be inferred.
The chief criterion for evaluating explanations is relevance. The relevance of an explanation corresponds to the cogency of the argument by which the fact to be explained is inferred from the proposed explanation. Any acceptable explanation must be relevant. The additional criteria is that the explanation be true. A relevant, but false, explanation does not count for anything.
Most scientific explanations are general rather than particular, however, few scientific explanations are directly verifiable as true (or false). Thus, the proposed requirement of truth is not directly applicable as a criterion for evaluating scientific theories.
Every explanation in science is put forth tentatively and provisionally. Every scientific explanation is regarded as a hypothesis, more or less probable on the basis of the available facts or relevant evidence.
For every hypothesis, the question of its truth or falsehood is open. The word "evidence" is used to refer to experience. Science is empirical; it holds that experience or observation is the test of truth. Hence, it is the essence of a scientific explanation that it be testable by observation.
If a scientific explanation is of a general nature, it can be tested indirectly. If the conclusions are found to be true, that provides some evidence for the truth of the proposition under question, which is thus confirmed indirectly.
A scientific explanation has directly testable propositions deducible from it, but an unscientific explanation has none. Scientific explanations are empirically verifiable (or refutable); unscientific explanations are not.
13.3 Evaluating Scientific Explanations
The three most commonly used criteria for judging the merit of competing scientific hypotheses are the following:
A. Compatibility with previously well-established hypotheses
B. Predictive or explanatory power
C. Simplicity
13.3A Compatibility with Previously Well-established Hypotheses
Science seeks to encompass more and more facts by creating a system of explanatory hypotheses. This system must be a set of self-consistent, noncontradictory propositions. Ideally, each new hypothesis must be consistent with those already confirmed. If there is to be orderly progress in the scientific enterprise, new hypotheses must fit with older ones. Every established theory has been accepted because it explained certain facts. It cannot be easily rejected unless a new hypothesis is able to account for the same set of facts in a better way. When there is a conflict between two hypotheses scientists turn to empirical observation to decide which is better. Since the totality of hypotheses accepted at any one time should be consistent with each other, when two hypotheses compete the one that fits in better with the complete body of scientific theories is, at least temporarily, to be preferred.
13.3B Predictive or Explanatory Power
The predictive or explanatory power of a hypothesis refers to the range of observable facts that can be deduced from it. If one of two testable hypotheses has a greater number of observable facts deducible from it than from the other, then it has greater predictive or explanatory power. An observable fact that can be deduced from a hypothesis is explained by it, and also predicted by it. Therefore, the greater the predictive power of a hypothesis the more it explains.
13.3C Simplicity
Two rival hypotheses may be relevant and testable, may fit equally well with established theory, and may even have roughly equal predictive power. In such circumstances we are likely to favor the simpler of the two—the one that requires the fewest assumptions, for example. But "simplicity" is a tricky notion. Two competing theories may each be simpler in different ways. Simplicity is an important criterion, even sometimes a decisive one—but it is difficult to formulate and not always easy to apply.
13.4 Seven Stages of Scientific Investigation
The general pattern of scientific research, or the scientific method, can be broken down into seven stages. Although we can distinguish them in the abstract, in actual scientific practice they overlap and impinge on each other. The seven stages of the scientific method are as follows:
13.4A Identifying the Problem
Scientific investigation begins when the investigator is confronted with something that needs explaining. A fact, or set of facts, for which we have no good explanation, is a problem.
13.4B Devising Preliminary Hypotheses
A preliminary hypothesis is needed to even begin deciding where to look for evidence. Every scientific investigator must rely on some prior set of beliefs or theories.
13.4C Collecting Additional Facts
13.4D Formulating the Explanatory Hypothesis
13.4E Deducing Further Consequences
13.4F Testing the Consequences
13.4G Applying the Theory
13.5 Scientists in Action: The Pattern of Scientific Investigation
The problem. Biologists wondered how genetic messages are conveyed from one generation to the next. Cells are made up of fats, sugars and starches, proteins, and nucleic acids. By 1944, research had enabled scientists to eliminate fats and sugars and starches as the medium for genetic messages. Many thought that, because of the extraordinary complexity of the genetic messages and the enormous detail and specificity that had to be conveyed, the secret of the gene could lie only in some large and very complicated protein. Subsequent research based on this assumption, however, proved fruitless.
Preliminary hypotheses. The failure of the protein search prompted James Watson and Francis Crick to pursue the nucleic acid path. Their general preliminary hypothesis was that the genetic message was somehow carried in the structure of deoxyribonucleic acid, or DNA. Based on previous research on DNA, they further hypothesized that that structure took the form of a spiral or helix.
Collecting additional facts. Nucleic acids were known to have a long "backbone" or "chain" consisting of a sugar, alternating with a phosphate, and with a third molecular unit, called a base, somehow, stuck onto the chain. Each three-piece unit in the chain was called a nucleotide. The general problem of genetic messages was thus reduced to the specific problem of how the nucleotides fit together to form the acid known as DNA.
Formulating the refined explanatory hypothesis. The final hypothesis had to propose a three-dimensional structure for DNA, consistent with known facts and theories, that could provide the coding for all the detail of life, and that could replicate itself generation after generation. Through calculation and manipulation of models, Watson and Crick determined that the constituents of DNA bonded in a chain of matching pairs of the bases adanine, guanine, cytosine, and thymine. This chain, split down the middle, could provide an elegant self-replicating mechanism. Each side of the chain could be viewed as a lock to which the other side was a matching key. And if the chain of matching pairs was long enough, the order and number of matching pairs could explain the required genetic coding needed to provide genetic detail. The solution, they hypothesized, was a double helix in which the bases bonded in complimentary matching pairs.
Deducing and testing consequences. The first deduction from the hypothesis was an obvious one. It should be possible to construct a three-dimensional model in which the bases would fit together internally, and the angles of the spiral as well as the other features of the chain would satisfy the requirements established by earlier studies. This was soon accomplished. For every additional theoretical deduction the subsequent testing proved successful. The hypothesis was well confirmed.
Application. Watson and Crick’s theory of the structure and function of DNA has had far-reaching impact, revolutionizing biology and medicine. Techniques for cutting and recombining DNA chains are used in the manufacture of drugs and vaccines. And thanks to research that followed from their crucial insight, a complete map of the entire human genome is now available.
13.6 Crucial Experiments and Ad Hoc Hypotheses
13.6A Crucial Experiments
Once a new hypothesis has been formulated, if it is inconsistent with some previously accepted theory, it may be difficult to determine which of the alternative accounts is correct. In some cases, two (or more) competing hypotheses may be tested by means of a crucial experiment, an experiment whose outcome is claimed to establish the falsehood of one of two competing and inconsistent scientific hypotheses. We cannot, however, always conduct a crucial experiment. Sometimes observable consequences may not be presently deducible from the new hypothesis; or they might be deducible, but we may lack the technological capacity to test the consequences.
More important, the consequences of some explanatory hypotheses cannot be deduced from the hypothesis alone. We can deduce the consequence to be tested only by applying the hypothesis together with others, which are assumed to be reliable. Since one (or more) of those other hypotheses may in fact be false, any experiment that seems to disconfirm the new hypothesis may rather reflect a flaw in some other hypothesis in the total set of hypotheses, of which the new one is an extension.
Where hypotheses of a fairly high level of abstraction are involved, no directly testable prediction can be deduced from just a single one of them. If a set of hypotheses is used as premisses for the deduction of a consequence, and if the observed facts are not as predicted, then we may conclude that at least one of the hypotheses in the set is false. But at this point we will not have established which one (or ones) is false. This result has led some to conclude that perhaps no individual hypothesis can ever be subjected to a crucial experiment.
13.6B Ad Hoc Hypotheses
Ad hoc has several meanings. A hypothesis may, for example, be ad hoc in the unobjectionable sense that, like all hypotheses, it was constructed to account for some antecedently established fact or other. More commonly, however ad hoc is applied pejoratively to describe a hypothesis constructed only for the purpose of saving a new hypothesis being tested, and that it has no other explanatory power or testable consequences. In addition, if a hypothesis is introduced to explain certain facts, and it cannot explain anything else, nor can it be further tested, then it is ad hoc. A hypothesis that is ad hoc in this sense is merely descriptive, it has no further explanatory power or scope, and it is not fruitful.
13.7 Classification as Hypothesis
It has been thought by some scientists that hypotheses play an important role only in the more advanced sciences such as physics, chemistry, and astronomy. Perhaps biology, history, and the social sciences are merely descriptive. But an examination of the nature of "description" shows this to be incorrect. A historian’s description of the past is a particular hypothesis, one that is intended to account for the present data that exist about the past. The historian’s data constitute evidence to support the particular hypothesis. Historians use the methods of science when sifting and sorting evidence to support or refute their particular descriptions of the past.
Biologists’ descriptions of data are not causal, but they are systematic. Classification and description go hand in hand. To describe a given animal is to classify it; to classify it is to describe it. Classification can be practical or theoretical.