An introduction to mathematics, by A. N. Whitehead.

CHAPTER XII
PERIODICITY IN NATURE
THE whole life of Nature is dominated by
the existence of periodic events, that is, by
the existence of successive events so analogous
to each other that, without any straining of
language, they may be termed recurrences of
the same event. The rotation of the earth
produces the successive days. It is true that
each day is different from the preceding days,
however abstractly we define the meaning of
a day, so as to exclude casual phenomena.
But with a sufficiently abstract definition of
a day, the distinction in properties between
two days becomes faint and remote from
practical interest; and each day may then
be conceived as a recurrence of the phenomenon of one rotation of the earth. Again the
path of the earth round the sun leads to the
yearly recurrence of the seasons, and imposes
another periodicity on all the operations of
nature. Another less fundamental periodicity is provided by the phases of the moon.
In modern civilized life, with its artificial light,
these phases are of slight importance, but in
164