annot at the moment answer that kind of reasoning. You
feel that your worthy friend has you somewhat at a disadvantage. You
will feel perfectly convinced in your own mind, however, that you are
quite right, and you say to him, "My good friend, I can only be guided
by the natural probabilities of the case, and if you will be kind enough
to stand aside and permit me to pass, I will go and fetch the police."
Well, we will suppose that your journey is successful, and that by good
luck you meet with a policeman; that eventually the burglar is found
with your property on his person, and the marks correspond to his hand
and to his boots. Probably any jury would consider those facts a very
good experimental verification of your hypothesis, touching the cause
of the abnormal phenomena observed in your parlor, and would act
accordingly.

Now, in this supposititious case, I have taken phenomena of a very
common kind, in order that you might see what are the different steps in
an ordinary process of reasoning, if you will only take the trouble to
analyse it carefully. All the operations I have described, you will
see, are involved in the mind of any man of sense in leading him to
a conclusion as to the course he should take in order to make good a
robbery and punish the offender. I say that you are led, in that case,
to your conclusion by exactly the same train of reasoning as that which
a man of science pursues when he is endeavouring to discover the origin
and laws of the most occult phenomena. The process is, and always must
be, the same; and precisely the same mode of reasoning was employed by
Newton [90] and Laplace [91] in their endeavours to discover and define
the causes of the movements of the heavenly bodies, as you, with your
own common sense, would employ to detect a burglar. The only difference
is, that the nature of the inquiry being more abstruse, every step has
to be most carefully watched, so that there may not be a single crack
or flaw in your hypothesis. A flaw or crack in many of the hypotheses
of daily life may be of little or no moment as affecting the general
correctness of the conclusions at which we may arrive; but, in a
scientific inquiry, a fallacy, great or small, is always of importance,
and is sure to be in the long run constantly productive of mischievous
if not fatal results.

Do not allow yourselves to be misled by the common notion that an
hypothesis is untrustworthy simply because it is an hypoth