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the hand of intelligence, and have not got their shape from 
natural causes, is a conclusion which a trained geologist alone 
would be competent to form. In some cases the recognition of 
a fact may require the highest skill and knowledge. It is not in 
the power of any one to use a microscope — the blundering hand 
of a neophyte may scarcely be able to present to the eye the 
commonest object with an instrument which in other hands 
may suffice to reveal the deepest secrets of that mysterious 
organism which has hitherto been found to pervade all matter. 
It is not the magnitude of the telescope, but the skill of the 
user of it, which brings the secrets of the heavens within 
human reach. The aberration which caught the notice of the 
astronomer Adams, and led to the discovery of new planets, 
was no fresh fact, yet none had discovered it till then. The 
observation of facts tasks, therefore, mental powers of high 
character, and can only be effectually done when a natural 
gift is developed by incessant practice into an exquisite 
mental sensibility. There is needed in addition the genius 
which can grasp the value of the fact, and by a rapid intuition 
seize its meaning. The steam of the kettle which led to the 
discovery of the steam-engine, the fall of the apple which 
suggested the law of gravitation, had been watched by count- 
less thousands of eyes before those of Newton and W att. Then, 
moreover, a fresh process of rigid examination is needed to 
eliminate possible causes of error. Those who remember the 
first outbreak of the table-turning mania may find an illustra- 
tion in that ridiculous epidemic. That tables turned was a 
fact patent enough. Faraday proved that their turning by a 
physical impulse was a fact likewise, but till his practised 
habit or experimental observation was brought to bear upon 
it, fear and wonder and superstition had magnified one of the 
simplest of facts into one of the most inexplicable of miracles. 
And lastly, when single facts Lave nearly been ascertained 
and valued, and possible causes of mistake eliminated, there 
is still needed a wide aggregation of facts before any general 
conclusion can be justified by them. The whole world must 
be ransacked, and it is hard to say at what point the search 
must end, or when it is possible to pronounce that no fresh 
and unexpected facts will suddenly turn up to destroy the 
conclusions founded on the old. This has taken place over and 
over again — so repeatedly that the experience of the past 
teaches the most excessive modesty and caution in the future. 
Little more can be said than that in the present state of our 
knowledge, that is, of our acquaintance with facts, such and 
such things are probably true. 
After saying thus much I shall not be suspected of under- 
