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exactly the same as that by which we have established the trivial
proposition about the sourness of hard and green apples。 But we
believe it in such an extensive； thorough； and unhesitating manner
because the universal experience of mankind verifies it； and we can
verify it ourselves at any time； and that is the strongest possible
foundation on which any natural law can rest。
So much by way of proof that the method of establishing laws in science
is exactly the same as that pursued in common life。 Let us now turn to
another matter （though really it is but another phase of the same
question）； and that is； the method by which； from the relations of
certain phenomena； we prove that some stand in the position of causes
towards the others。
I want to put the case clearly before you； and I will therefore show you
what I mean by another familiar example。 I will suppose that one of
you； on coming down in the morning to the parlour of your house； finds
that a tea…pot and some spoons which had been left in the room on the
previous evening are gone；the window is open； and you observe the mark
of a dirty hand on the window…frame； and perhaps； in addition to that；
you notice the impress of a hob…nailed shoe on the gravel outside。 All
these phenomena have struck your attention instantly； and before two
minutes have passed you say； 〃Oh； somebody has broken open the window；
entered the room； and run off with the spoons and the tea…pot！〃 That
speech is out of your mouth in a moment。 And you will probably add； 〃I
know there has； I am quite sure of it！〃 You mean to say exactly what
you know； but in reality what you have said has been the expression of
what is； in all essential particulars； an Hypothesis。 You do not 'know'
it at all； it is nothing but an hypothesis rapidly framed in your own
mind！ And it is an hypothesis founded on a long train of inductions
and deductions。
What are those inductions and deductions； and how have you got at this
hypothesis？ You have observed； in the first place； that the window is
open； but by a train of reasoning involving many Inductions and
Deductions； you have probably arrived long before at the General
Lawand a very good one it isthat windows do not open of themselves；
and you therefore conclude that something has opened the window。 A
second general law that you have arrived at in the same way is； that
tea…pots and spoons do not go out of a window spontaneously； and you
are satisfied that； as they are not now where you left them； they have
been removed。 In the third place； you look at the marks on the
window…sill； and the shoemarks outside； and you say that in all
previous experience the former kind of mark has never been produced by
anything else but the hand of a human being； and the same experience
shows that no other animal but man at present wears shoes with
hob…nails on them such as would produce the marks in the gravel。 I do
not know； even if we could discover any of those 〃missing links〃 that
are talked about； that they would help us to any other conclusion！ At
any rate the law which states our present experience is strong enough
for my present purpose。You next reach the conclusion； that as these
kinds of marks have not been left by any other animals than men； or are
liable to be formed in any other way than by a man's hand and shoe； the
marks in question have been formed by a man in that way。 You have；
further； a general law； founded on observation and experience； and
that； too； is； I am sorry to say； a very universal and unimpeachable
one；that some men are thieves； and you assume at once from all these
premissesand that is what constitutes your hypothesisthat the man
who made the marks outside and on the window…sill； opened the window；
got into the room； and stole your tea…pot and spoons。 You have now
arrived at a 'Vera Causa'；you have assumed a Cause which it is plain
is competent to produce all the phenomena you have observed。 You can
explain all these phenomena only by the hypothesis of a thief。 But
that is a hypothetical conclusion； of the justice of which you have no
absolute proof at all； it is only rendered highly probable by a series
of inductive and deductive reasonings。
I suppose your first action； assuming that you are a man of ordinary
common sense； and that you have established this hypothesis to your own
satisfaction； will very likely be to go off for the police； and set
them on the track of the burglar； with the view to the recovery of your
property。 But just as you are starting with this object； some person
comes in； and on learning what you are about； says； 〃My good friend；
you are going on a great deal too fast。 How do you know that the man
who really made the marks took the spoons？ It might have been a monkey
that took them； and the man may have merely looked in afterwards。〃 You
would probably reply； 〃Well； that is all very well； but you see it is
contrary to all experience of the way tea…pots and spoons are
abstracted； so that； at any rate； your hypothesis is less probable than
mine。〃 While you are talking the thing over in this way； another friend
arrives； one of that good kind of people that I was talking of a little
while ago。 And he might say； 〃Oh； my dear sir； you are certainly going
on a great deal too fast。 You are most presumptuous。 You admit that
all these occurrences took place when you were fast asleep； at a time
when you could not possibly have known anything about what was taking
place。 How do you know that the laws of Nature are not suspended
during the night？ It may be that there has been some kind of
supernatural interference in this case。〃 In point of fact； he declares
that your hypothesis is one of which you cannot at all demonstrate the
truth； and that you are by no means sure that the laws of Nature are
the same when you are asleep as when you are awake。
Well； now； you cannot 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 parlour； and would act
accordingly。
Now； in this suppositious 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