sciences. The phenomena
of light are not yet subject to accurate measurement;
many natural phenomena have not yet been made the subject
of measurement at all. Such are the intensity of sound, the
phenomena of taste and smell, the magnitude of atoms, the
temperature of the electric spark or of the sun's atmosphere.[1]

[Footnote 1: See Jevons, p. 273.]

The sciences tend, in general, to become more and more
quantitative. All phenomena "exist in space and involve
molecular movements, measurable in velocity and extent."
The ideal of all sciences is thus to reduce all phenomena to
measurements of mass and motion. This ideal is obviously
far from being attained. Especially in the social sciences
are quantitative measurements difficult, and in these sciences
we must remain therefore at best in the region of shrewd
guesses or fairly reliable probability.

STATISTICS AND PROBABILITY. While in the social sciences,
exact quantitative measurements are difficult, they are to an
extent possible, and to the extent that they are possible we
can arrive at fairly accurate generalizations as to the probable
occurrence of phenomena. There are many phenomena where
the elements are so complex that they cannot be analyzed
and invariable causal relations established.

In a study of the phenomena of the weather, for example, the phenomena
are so exceedingly complex that anything approaching a
complete statement of their elements is quite out of the question.
The fallibility of most popular generalizations in these fields is
evidence of the difficulty of dealing with such facts. Must we be
content then simply to guess at such phenomena? ... In instances of
this sort, another method ... becomes important: The Method of
Statistics. In statistics we have an _exact_ enumeration of cases. If
a small number of cases does not enable us to detect the causal
relations of a phenomenon, it sometimes happens that a large number,
accurately counted, and taken from a field widely extended in time
and space, will lead to a solution of the problem.[1]

[Footnote 1: Jones; _Logic, Inductive and Deductive_, p. 190.]

If we find, in a wide variety of instances, two phenomena
occurring in a certain constant correlation, we infer a causal
relation. If the variations in the frequency of one correspond
to variations in the frequency of the other, there is probability
of more than connection by coincidence.

The correlation between phenomena may be measured