ysical objects, may be expected under ordinary circumstances to
behave, tells us what we may in a general way expect of any individual
member of that class. If natural science seeks to predict, it is able to
do so simply because it operates with concepts or class names instead,
as is the case with history, with concrete facts and, to use a logical
phrase, "existential propositions."

That the chief end of science is descriptive formulation has
probably been clear to keen analytic minds since the time of
Galileo, especially to the great discoverers in astronomy,
mechanics, and dynamics. But as a definitely stated conception,
corrective of misunderstandings, the view of science as
essentially descriptive began to make itself felt about the
beginning of the last quarter of the nineteenth century, and
may be associated with the names of Kirchhoff and Mach. It was
in 1876 that Kirchhoff defined the task of mechanics as that of
"describing completely and in the simplest manner the motions
which take place in nature." Widening this a little, we may say
that the aim of science is to describe natural phenomena and
occurrences as exactly as possible, as simply as possible, as
completely as possible, as consistently as possible, and always
in terms which are communicable and verifiable. This is a very
different role from that of solving the riddles of the
universe, and it is well expressed in what Newton said in
regard to the law of gravitation: "So far I have accounted for
the phenomena presented to us by the heavens and the sea by
means of the force of gravity, but I have as yet assigned no
cause to this gravity.... I have not been able to deduce from
phenomena the _raison d'etre_ of the properties of gravity and
I have not set up hypotheses." (Newton, _Philosophiae naturalis
principia Mathematica_, 1687.)

"We must confess," said Prof. J. H. Poynting (1900, p. 616),
"that physical laws have greatly fallen off in dignity. No long
time ago they were quite commonly described as the Fixed Laws
of Nature, and were supposed sufficient in themselves to govern
the universe. Now we can only assign to them the humble rank of
mere descriptions, often erroneous, of similarities which we
believe we have observed.... A law of nature explains nothing,
it has no governing power, it is