Foreknowledge; here, too, there is a contrast with the great mass of consistent and reconcilable things.  But now, when we are told by sensational writers, that everything is mysterious; that the simplest phenomenon in nature—­the fall of a stone, the swing of a pendulum, the continuance of a ball shot in the air—­are wonderful, marvellous, miraculous, our understanding is confounded; there being then nothing plain at all, there is nothing mysterious.  The wonderful rises from the common; as the lofty is lofty by relation to something lower:  if there is nothing common, then there is nothing wonderful; if all phenomena are mysterious, nothing is mysterious; if we are to stand aghast in amazement because three times four is twelve, what phenomenon can we take as the type of the plain and the intelligible?  You must always keep up a standard of the common, the easy, the comprehensible, if you are to regard other things as wonderful, difficult, inexplicable.

[LOCKE ON THE LIMITS OF THE UNDERSTANDING.]

The real 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.