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reasoning is what logicians call a syllogism； and has all its
various parts and terms；its major premiss； its minor premiss and
its conclusion。 And； by the help of further reasoning； which； if
drawn out； would have to be exhibited in two or three other
syllogisms； you arrive at your final determination； 〃I will not
have that apple。〃 So that； you see； you have； in the first place；
established a law by induction； and upon that you have founded a
deduction； and reasoned out the special particular case。 Well now；
suppose； having got your conclusion of the law； that at some time
afterwards； you are discussing the qualities of apples with a
friend： you will say to him； 〃It is a very curious thing；but I
find that all hard and green apples are sour！〃 Your friend says to
you； 〃But how do you know that？〃 You at once reply； 〃Oh； because I
have tried them over and over again； and have always found them to
be so。〃 Well； if we were talking science instead of common sense；
we should call that an experimental verification。 And； if still
opposed； you go further； and say； 〃I have heard from the people in
Somersetshire and Devonshire； where a large number of apples are
grown； that they have observed the same thing。 It is also found to
be the case in Normandy； and in North America。 In short； I find it
to be the universal experience of mankind wherever attention has
been directed to the subject。〃 Whereupon； your friend； unless he
is a very unreasonable man； agrees with you； and is convinced that
you are quite right in the conclusion you have drawn。 He believes；
although perhaps he does not know he believes it； that the more
extensive verifications are；that the more frequently experiments
have been made； and results of the same kind arrived at；that the
more varied the conditions under which the same results are
attained； the more certain is the ultimate conclusion； and he
disputes the question no further。 He sees that the experiment has
been tried under all sorts of conditions； as to time； place； and
people； with the same result； and he says with you； therefore； that
the law you have laid down must be a good one； and he must believe
it。
In science we do the same thing；the philosopher exercises
precisely the same faculties； though in a much more delicate
manner。 In scientific inquiry it becomes a matter of duty to
expose a supposed law to every possible kind of verification； and
to take care； moreover； that this is done intentionally； and not
left to a mere accident； as in the case of the apples。 And in
science； as in common life； our confidence in a law is in exact
proportion to the absence of variation in the result of our
experimental verifications。 For instance； if you let go your grasp
of an article you may have in your hand； it will immediately fall
to the ground。 That is a very common verification of one of the
best established laws of naturethat of gravitation。 The method
by which men of science establish the existence of that law is
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； then； 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 parlor 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 seconds 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 you are giving expression to 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 windowsill； and the shoe…marks
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 in 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 kind ＇89＇ of marks have not been left by any other animal than
man； or are liable to be formed in any other way than 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 the 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 an