just as all the motions of the planets are (but--are they?) the direct
results of the single law of gravitation. Gravitation will, probably,
soon be explained in terms of some remoter cause, but the reason of
that single and ultimate law of the universe which we have imagined
would still remain unknown. Human knowledge will always have limits,
and beyond those limits there will always be room for mystery and
wonder. A complete and exhaustive explanation of the world is
inconceivable, so long as human powers and capacities remain at all as
they now are.

It is important to emphasize such truths, especially in a book
addressed to the young. When a lad hears for the first time that an
astronomer, by a simple pointing of his spectroscope, can determine
with what velocity a star is approaching the earth, or receding from
it, or when he hears that the very shape of the revolving masses of
certain stars can be calculated from simple measures of the sort, he
is apt to conclude that Science, which has made such astounding
advances since the days of Galileo and Newton, must eventually reach a
complete explanation of the entire universe. The conclusion is not
unnatural, but it is not correct. There are limits beyond which
Science, in this sense, cannot go. Its scope is limited. Beyond its
limits there are problems that it cannot solve, mysteries that it
cannot explain.

At the present moment, for example, the nature of Force is unknown. A
weight released from the hand drops to the earth. Exactly what is the
nature of the force with which the earth attracts it? We do not know,
but it so happens that it is more than likely that an explanation will
be reached in our own day. Gravity will be explained in terms of some
more general forces. The mystery will be pushed back another step, and