问答题
Laws of nature are of two basic forms: (1) a law is
universal if it states that some conditions, so far as are known, invariably are
found together with certain other conditions; and (2) a law is probabilistic if
it affirms that, on the average, a stated fraction of cases displaying a given
condition will display a certain other condition as well. In either ease, a law
may be valid even though it obtains only under special circumstances or as a
convenient approximation. {{U}} {{U}} 1 {{/U}}
{{/U}}{{U}}Moreover, a law of nature has no logical necessity; rather, it rests
directly or indirectly upon the evidence of experience.{{/U}}
Laws of universal form must be distinguished from generalizations, such as "All
chairs in this office are gray," which appear to be accidental. Generalizations,
for example, cannot support counterfactual conditional statements such as "If
this chair had bees in my office, it would be gray" nor subjunctive conditionals
such as "If this chair were put in my office, it would be gray." On the other
hand, the statement "All planetary objects move in nearly elliptical paths about
their star" does provide this support. All scientific laws appear to give
similar results. {{U}} {{U}} 2 {{/U}} {{/U}}{{U}}The class of
universal statements that can be candidates for the status of laws, however, is
determined at any time in history the theories of science then
current.{{/U}} Several positive attributes are commonly required
of a natural law. Statements about things or events limited to one location or
one date cannot be lawlike. Also, most scientists hold that the predicate must
apply to evidence not used in deriving the law: though the law is founded upon
experience, it must predict or help one to understand matters not included among
these experiences. Finally, it is normally expected that a law will be
explainable by more emhracing laws or by some theory. {{U}} {{U}}
3 {{/U}} {{/U}}{{U}}Thus, a regularity for which there are general
theoretical grounds for expecting it will be more readily called a natural law
than an empirical regularity that cannot be subsumed under more general laws or
theories.{{/U}} Universal laws are of several types. {{U}}
{{U}} 4 {{/U}} {{/U}}{{U}}Many assert a dependence between varying
quantities measuring certain properties, as in the law that the pressure of a
gas under steady temperature is inversely proportional to its volume.{{/U}} Others
state that events occur in an invariant order, as in "Vertebrates always occur
in the fossil record after the rise of invertebrates." Lastly, there are laws
affirming that if an object is of a stated sort it will have certain observable
properties. {{U}} {{U}} 5 {{/U}} {{/U}}{{U}}Part of the reason for
the ambiguity of the term law of nature lies in the temptation to apply the term
only to statements of one of these sorts of laws, as in the claim that science
deals solely with cause and effect relationships, when in fact all three kinds
are equally valid.{{/U}}