And geology, which traces back the course of
history beyond the limits of archaeology, could tell us nothing
except for the assumption that, millions of years ago, water,
heat, gravitation, friction, animal and vegetable life, caused
effects of the same kind as they now cause. Nay, even physical
astronomy, in so far as it takes us back to the uttermost point
of time which palaetiological science can reach, is founded upon
the same assumption. If the law of gravitation ever failed to be
true, even to a small extent, for that period, the calculations
of the astronomer have no application.
The power of prediction, of prospective prophecy, is that which
is commonly regarded as the great prerogative of physical
science. And truly it is a wonderful fact that one can go into a
shop and buy for a small price a book, the "Nautical Almanac,"
which will foretell the exact position to be occupied by one of
Jupiter's moons six months hence; nay, more, that, if it were
worth while, the Astronomer-Royal could furnish us with as
infallible a prediction applicable to 1980 or 2980.
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.
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