n the midst
of known circumstances, the phenomena which Nature exhibits on
a grander scale in the form of lightning and thunder. Now let any
one consider what amount of knowledge of the effects and laws of
electric agency mankind could have obtained from the mere observation
of thunderstorms, and compare it with that which they have
gained, and may expect to gain, from electrical and galvanic
experiments....

When we have succeeded in isolating the phenomenon which is
the subject of inquiry, by placing it among known circumstances,
we may produce further variations of circumstances to any extent,
and of such kinds as we think best calculated to bring the laws of the
phenomenon into a clear light. By introducing one well-defined
circumstance after another into the experiment, we obtain assurance
of the manner in which the phenomenon behaves under an indefinite
variety of possible circumstances. Thus, chemists, after having
obtained some newly discovered substance in a pure state, ... introduce
various other substances, one by one, to ascertain whether it
will combine with them, or decompose them, and with what result;
and also apply heat or electricity or pressure, to discover what will
happen to the substance under each of these circumstances.[1]

[Footnote 1: Mill: _Logic_ (London, 1872), vol. I, pp. 441-42.]

Through experiment, we are thus enabled to observe the
relation of specific elements in a situation. We are, furthermore,
enabled to observe phenomena which are so rare in occurrence
that it is impossible to form generalizations from
them or improbable that we should even notice them: "We
might have to wait years or centuries to meet accidentally
with facts which we can readily produce at any moment in a
laboratory; and it is probable that many of the chemical
substances now known, and many excessively useful products,
would never have been discovered at all, by waiting till Nature
presented them spontaneously to our observation." And
phenomena, such as that of electricity, which can only be
understood when the conditions of their occurrence are varied,
are presented to us in Nature most frequently in a
fixed and invariable form.

GENERALIZATIONS, THEIR ELABORATION AND TESTING. So far
we have been concerned with the steps in the control of
suggestion, the reexamination of the facts so that significant
suggestions may be derived, and the elimination of the
significant from the insignificant in the elements