Testing a scientific hypothesis involves at least four
different activities. First, the hypothesis must be examined for
internal consistency. A hypothesis that is self-contradictory
or not logically well-formed in some other way should be rejected.
Second, the logical structure of the hypothesis must be examined
to ascertain whether it has explanatory value, i.e., whether it
makes the observed phenomena intelligible in some sense, whether
it provides an understanding of why the phenomena do in fact occur
A hypothesis that is purely tautological should be rejected
because it has no explanatory value. A scientific hypothesis identifies
the conditions, processes, or mechanisms that account for the
phenomena it purports to explain. Thus, hypotheses establish general
relationships between certain conditions and their consequences
or between certain causes and their effects. For example, the
motions of the planets around the sun are explained as a consequence
of gravity, and respiration as an effect of red blood cells that
carry oxygen from the lungs to various parts of the body.
Third, the hypothesis must be examined for its consistency
with hypotheses and theories commonly accepted in the particular
field of science, or to see whether it represents any advance
with respect to well-established alternative hypotheses. Lack
of consistency with other theories is not always ground for rejection
of a hypothesis, although it will often be. Some of the greatest
scientific advances occur precisely when it is shown that a widely-held
and well supported hypothesis is replaced by a new one that accounts
for the same phenomena that were explained by the preexisting
hypothesis, as well as other phenomena it could not account for.
One example is the replacement of Newtonian mechanics by the theory
of relativity, which rejects the conservation of matter and the
simultaneity of events that occur at a distancetwo fundamental
tenets of Newton's theory.
Examples of this kind are pervasive in rapidly advancing disciplines,
such as molecular biology at present. The so-called "central
dogma" holds that molecular information flows only in one
direction, from DNA to RNA to protein. The DNA contains the genetic
information that determines what the organism is, but that information
has to be expressed in enzymes (a particular class of proteins)
that guide all chemical processes in cells. The information contained
in the DNA molecules is conveyed to proteins by means of intermediate
molecules, called messenger RNA. David Baltimore and Howard Temin
were awarded the Nobel Prize for discovering that information
could flow in the opposite direction, from RNA to DNA, by means
of the enzyme reverse transcriptase. They showed that some viruses,
as they infect cells, are able to copy their RNA into DNA, which
then becomes integrated into the DNA of the infected cell, where
it is used as if it were the cell's own DNA.
Other examples are the following. Until very recently, it was
universally thought that only the proteins known as enzymes could
mediate (technically "catalyze") the chemical reactions
in cells. However, Thomas Cech and Sidney Altman received in 1989
the Nobel Prize for showing that certain RNA molecules act as
enzymes and catalyze their own reactions. One more example concerns
the so-called "co-linearity" between DNA and protein.
It was generally thought that the sequence of nucleotides in the
DNA of a gene is expressed consecutively in the sequence of aminoacids
in the protein. This conception was shaken by the discovery that
genes come in pieces, separated by intervening DNA segments that
do not carry genetic information; Richard Roberts and Philip Sharp
received the 1993 Nobel Prize for this discovery.
The fourth and most distinctive test is the one I have identified,
which consists of putting on trial an empirically scientific hypothesis
by ascertaining whether or not predictions about the world of
experience derived as logical consequences from the hypothesis
agree with what is actually observed. This is the critical element
that distinguishes the empirical sciences from other forms of
knowledge: the requirement that scientific hypotheses be empirically
falsifiable. Scientific hypotheses cannot be consistent with all
possible states of affairs in the empirical world. A hypothesis
is scientific only if it is consistent with some but not with
other possible states of affairs not yet observed in the world,
so that it may be subject to the possibility of falsification
by observation. The predictions derived from a scientific hypothesis
must be sufficiently precise that they limit the range of possible
observations with which they are compatible. If the results of
an empirical test agree with the predictions derived from a hypothesis,
the hypothesis is said to be provisionally corroborated; otherwise
it is falsified.
The requirement that a scientific hypothesis be falsifiable
has been called by Karl Popper the criterion of demarcation
of the empirical sciences because it sets apart the empirical
sciences from other forms of knowledge. A hypothesis that is not
subject to the possibility of empirical falsification does not
belong in the realm of science.
See my "On the Scientific Method, Its Practice and Pitfalls,"
Hist. Phil. Life Sci. 16 (1994), pp. 205-240.
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