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with yourselves； on the discovery that you have been acting on the
principles of inductive and deductive philosophy during the same
period。 Probably there is not one here who has not in the course
of the day had occasion to set in motion a complex train of
reasoning； of the very same kind； though differing of course in
degree； as that which a scientific man goes through in tracing the
causes of natural phenomena。
A very trivial circumstance will serve to exemplify this。 Suppose
you go into a fruiterer's shop； wanting an apple；you take up one；
and； on biting it； you find it is sour； you look at it； and see
that it is hard and green。 You take up another one； and that too
is hard； green； and sour。 The shopman offers you a third； but；
before biting it； you examine it； and find that it is hard and
green； and you immediately say that you will not have it； as it
must be sour； like those that you have already tried。
Nothing can be more simple than that； you think； but if you will
take the trouble to analyse and trace out into its logical elements
what has been done by the mind； you will be greatly surprised。 In
the first place you have performed the operation of induction。 You
found that； in two experiences； hardness and greenness in apples
went together with sourness。 It was so in the first case； and it
was confirmed by the second。 True； it is a very small basis； but
still it is enough to make an induction from； you generalise the
facts； and you expect to find sourness in apples where you get
hardness and greenness。 You found upon that a general law that all
hard and green apples are sour； and that； so far as it goes； is a
perfect induction。 Well； having got your natural law in this way；
when you are offered another apple which you find is hard and
green； you say； 〃All hard and green apples are sour； this apple is
hard and green； therefore this apple is sour。〃 That train of
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
w