The mode in which this can be done depends upon the fact that in nature we often find several causes, essential to produce an effect, connected together, as it were, in a chain; so that each link depends upon that which precedes it. Thus the power of vision depends upon the optic nerve, in the bottom of the eye. But this would be useless, were not the coats and humors of the eye of a certain consistence and curvature, in order to bring the rays together to form an image on the retina. Again, these coats and humors depend upon light, and light depends for its transmission, probably, upon that exceedingly elastic medium called the _luminiferous ether_. This is as far back as we can trace the series of causes concerned in producing vision. And yet this elastic ether may depend upon something else, and this cause of the movement of the ether upon another cause; and we know not how long the chain may be before we reach the great First Cause. Now, if any one of this series of second causes be modified, the effect will be a modification of the final result. This supposed modification may take place in that part of the chain of causes within our view, or in that part concealed from us. If it took place within sight, it would constitute a miracle; because the regular sequence of cause and effect would be broken off, or an unnatural power be imparted to the cause producing the ultimate effect. If the modification took place in that part of the chain of second causes out of our sight, the final effect would be no miracle; because it would be brought about by natural laws, and these would perfectly explain it. Nevertheless, this ultimate effect would be different from what it would be if God had not touched and modified that link of causation which lies out of our sight, back among the secret agencies of his will. And I see not but in this way he might modify the ultimate effect as much as he pleased, and still preserve the unvarying constancy of nature. For in all these cases we should see only the links of the chain of causes nearest to us; and, provided they operated in their usual order, how could we know that any change had taken place in the region beyond our knowledge? If the whole chain of causation were open to our inspection, then, indeed, would the transaction be an obvious miracle; but now we see nothing but the unchanging operation of natural laws.