The lesson imparted by Newton and Locke and their successors still remains to be carried out and embodied in the subtler inquiries.  The bearing upon what constitutes a Mystery, and what constitutes Explanation, or the accounting for appearances, may be expressed thus:—­

In the first place, the Understanding can never pass out of its own experience—­its acquired knowledge, whether of body or of mind.  What we obtain by our various sensibilities to the world about us, and by our self-consciousness, are the foundation, the ABC of everything that we are capable of knowing.  We know colours, and we know sound; we know pleasure and pain, and the various emotions of wonder, fear, love, anger.  If there be any being endowed with senses different from ours, with that being we can have no communion.  If there be any phenomena that escape our limited sensibilities, they transcend the possibility of our knowledge.

It is necessary, however, to take account of the combining or constructive aptitudes of the mind.  We can go a certain length in putting together our alphabet of sensation and experience into many various compounds.  We can imagine a paradise or a pandemonium; but only as made up of our own knowledge of things good and evil.  The limits of this constructive power are soon reached.  We are baffled to enter into the feelings of our own kindred, when they are far removed in character and circumstances from ourselves.  The youth at twenty cannot approximate to the feelings of men of middle age.  The healthy are unable to comprehend the life of the invalid.

[TIME AND SPACE RELATIVE TO OUR FACULTIES.]

To come to the practical applications.  The great leading notions called Time and Space are known to us only under the conditions of our own sensibility.  Time is made known by all our actions, all our senses, all our feelings, and by the succession of our thoughts; it is experienced as a continuance and a repetition of movement, sight, sound, fear, or any other state of feeling, or of thinking.  One motion or sensation is continued longer than another; or it is more frequently repeated after intermission, giving the numerical estimate of time, as in the beats of the pendulum.  In these ways we form estimates of seconds, minutes, hours, days.  And our constructive faculty can be brought into play to conceive the larger tracts of duration—­a century, or a hundred centuries.  Nay, by our arithmetical powers we can put down in cipher, or conceive symbolically (which is the meagrest of all conceptions) millions of millions of centuries; these being after all but compounds of our alphabet of enduring or repeated sensations and thoughts.  We can suppose this arithmetical process to operate upon past duration or upon future duration, and there is no limit to the numbers that we can write down.  But there is one thing that we cannot do; we cannot fix upon a point when Time or succession began, or upon a point when it will cease.