al character of a MYSTERY, and what constitutes the Explanation of
a fact, have been greatly misconceived. The changes of view on these
points make up a chapter in the history of the education of the human
mind. Perhaps the most decisive turning point was the publication of
Locke's "Essay concerning Human Understanding," the motive of which, as
stated in the homely and forcible language of the preface, was to
ascertain what our understandings can do, what subjects they are fit to
deal with, and where they should stop. I quote a few sentences:--

"If by this inquiry into the nature of the Understanding, I can discover
the powers thereof; how far they reach; to what things they are in any
degree proportionate; and where they fail us: I suppose it may be of
use, to prevail with the busy mind of man to be more cautious in
meddling with things exceeding its comprehension; to stop when it is at
the utmost extent of its tether; and to sit down in a quiet ignorance of
those things which, upon examination, are proved to be beyond the reach
of our capacities." "The candle that is set up in us, shines bright
enough for all our purposes. The discoveries we can make with this ought
to satisfy us. And we shall then use our Understandings aright, when we
entertain all objects in that way and proportion that they are suited to
our faculties, and upon those grounds they are capable of being proposed
to us." "It is of great use for the sailor to know the length of his
line, though he cannot fathom with it all the depths of the ocean."

The course of physical science was preparing the same salutary lesson.
Locke's great contemporary and friend, Isaac Newton, was his
fellow-worker in this tutorial undertaking; nor should Bacon be
forgotten, although there is dispute as to the extent and character of
his influence. The combined operation of these great leaders of thought
was apparent in the altered views of scientific inquirers as to what is
competent in research--what is the proper aim of inquiry. There arose a
disposition to abandon the pursuit of mysterious essences and grand
pervading unities, and ascertain with precision the facts and the laws
of natural phenomena. The study of astronomy was inaugurated in
Greenwich Observatory. The experiments of Priestley and of Franklin
farther exemplified the eighteenth-century key to the secrets of the
universe.

The lesson imparted by Newton and Locke and their successors still
remains to be ca