But astronomy is not less remarkable for its power of
retrospective prophecy.

Thales, oldest of Greek philosophers, the dates of whose
birth and death are uncertain, but who flourished about 600
B.C., is said to have foretold an eclipse of the sun which took
place in his time during a battle between the Medes and the
Lydians. Sir George Airy has written a very learned and
interesting memoir<2> in which he proves that such an eclipse
was visible in Lydia on the afternoon of the 28th of May in the
year 585 B.C.

No one doubts that, on the day and at the hour mentioned by the
Astronomer-Royal, the people of Lydia saw the face of the sun
totally obscured. But, though we implicitly believe this
retrospective prophecy, it is incapable of verification. In the
total absence of historical records, it is impossible even to
conceive any means of ascertaining directly whether the eclipse
of Thales happened or not. All that can be said is, that the
prospective prophecies of the astronomer are always verified;
and that, inasmuch as his retrospective prophecies are the
result of following backwards, the very same method as that
which invariably leads to verified results, when it is worked
forwards, there is as much reason for placing full confidence in
the one as in the other. Retrospective prophecy is therefore a
legitimate function of astronomical science; and if it is
legitimate for one science it is legitimate for all;
the fundamental axiom on which it rests, the constancy of the
order of nature, being the common foundation of all scientific