Such are a few of the new conceptions implanted in our minds by the improvement of natural knowledge.  Men have acquired the ideas of the practically infinite extent of the universe and of its practical eternity; they are familiar with the conception that our earth is but an infinitesimal fragment of that part of the universe which can be seen; and that, nevertheless, its duration is, as compared with our standards of time, infinite.  They have further acquired the idea that man is but one of innumerable forms of life now existing on the globe, and that the present existences are but the last of an immeasurable series of predecessors.  Moreover, every step they have made in natural knowledge has tended to extend and rivet in their minds the conception of a definite order of the universe—­which is embodied in what are called, by an unhappy metaphor, the laws of Nature—­and to narrow the range and loosen the force of men’s belief in spontaneity, or in changes other than such as arise out of that definite order itself.

Whether these ideas are well or ill founded is not the question.  No one can deny that they exist, and have been the inevitable outgrowth of the improvement of natural knowledge.  And if so, it cannot be doubted that they are changing the form of men’s most cherished and most important convictions.

And as regards the second point—­the extent to which the improvement of natural knowledge has remodelled and altered what may be termed the intellectual ethics of men,—­what are among the moral convictions most fondly held by barbarous and semi-barbarous people?

They are the convictions that authority is the soundest basis of belief; that merit attaches to a readiness to believe; that the doubting disposition is a bad one, and scepticism a sin; that when good authority has pronounced what is to be believed, and faith has accepted it, reason has no further duty.  There are many excellent persons who yet hold by these principles, and it is not my present business, or intention, to discuss their views.  All I wish to bring clearly before your minds is the unquestionable fact, that the improvement of natural knowledge is effected by methods which directly give the lie to all these convictions, and assume the exact reverse of each to be true.

The improver of natural knowledge absolutely refuses to acknowledge authority, as such.  For him, scepticism is the highest of duties; blind faith the one unpardonable sin.  And it cannot be otherwise, for every great advance in natural knowledge has involved the absolute rejection of authority, the cherishing of the keenest scepticism, the annihilation of the spirit of blind faith; and the most ardent votary of science holds his firmest convictions, not because the men he most venerates hold them; not because their verity is testified by portents and wonders; but because his experience teaches him that whenever he chooses to bring these convictions into contact with their primary source, Nature—­whenever he thinks fit to test them by appealing to experiment and to observation—­Nature will confirm them.  The man of science has learned to believe in justification, not by faith, but by verification.