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.