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The method of scientific investigation is nothing but the expression of
the necessary mode of working of the human mind。 It is simply the mode
at which all phenomena are reasoned about； rendered precise and exact。
There is no more difference； but there is just the same kind of
difference； between the mental operations of a man of science and those
of an ordinary person； as there is between the operations and methods
of a baker or of a butcher weighing out his goods in common scales； and
the operations of a chemist in performing a difficult and complex
analysis by means of his balance and finely…graduated weights。 It is
not that the action of the scales in the one case； and the balance in
the other； differ in the principles of their construction or manner of
working； but the beam of one is set on an infinitely finer axis than
the other； and of course turns by the addition of a much smaller
weight。
You will understand this better； perhaps； if I give you some familiar
example。 You have all heard it repeated； I dare say； that men of
science work by means of Induction and Deduction； and that by the help
of these operations； they； in a sort of sense； wring from Nature
certain other things； which are called Natural Laws； and Causes； and
that out of these； by some cunning skill of their own； they build up
Hypotheses and Theories。 And it is imagined by many； that the
operations of the common mind can be by no means compared with these
processes； and that they have to be acquired by a sort of special
apprenticeship to the craft。 To hear all these large words； you would
think that the mind of a man of science must be constituted differently
from that of his fellow men； but if you will not be frightened by
terms； you will discover that you are quite wrong； and that all these
terrible apparatus are being used by yourselves every day and every
hour of your lives。
There is a well…known incident in one of Moliere's plays； where the
author makes the hero express unbounded delight on being told that he
had been talking prose during the whole of his life。 In the same way；
I trust； that you will take comfort； and be delighted 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 analyze 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 go 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 generalize 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 conclusion of the particular case。 Well now； suppose；
having got your 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 it 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 have been 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 o