stinctions or
differences. In scientific inquiry, differences are as crucial in
the forming of generalizations as similarities. It is only possible
to classify a given fact under a scientific generalization
when the given fact is set off from other facts, when it is seen
to be the result of certain special conditions.

If a man infers from a single sample of grain as to the grade of
wheat of the car as a whole, it is induction, and under certain
circumstances, a _sound_ induction; other cases are resorted to simply for the
sake of rendering that induction more guarded and correct. In the
case of the various samples of grain, it is the fact that the samples
are unlike, at least in the part of the carload from which they are
taken, that is important. Were it not for this unlikeness, their likeness
in quality would be of no avail in assisting inference.[1]

[Footnote 1: Dewey: _How We Think_, pp. 89-90.]

EXPERIMENTAL VARIATION OF CONDITIONS. In forming our
generalizations from the observation of situations as they occur
in Nature, we are at a disadvantage. If we observe cases
just as we find them, there is much present that is irrelevant
to our problem; much that is of genuine importance in its
solution is hidden or obscure. In experimental investigation
we are, in the words of Sir John Herschel, "active observers";
we deliberately invent crucial or test cases. That is, we
deliberately arrange conditions so that every factor is definitely
known and recognized. We then introduce into this set of
completely known conditions one change, one new circumstance,
and observe its effect. In Mill's phrase, we "take a
phenomenon home with us," and watch its behavior. Mill
states clearly the outstanding advantage of experimentation
over observation:

When we can produce a phenomenon artificially, we can take it, as
it were, home with us, and observe it in the midst of circumstances
with which in all other respects we are accurately acquainted. If
we desire to know what are the effects of the cause _A_, and are able to
produce _A_ by means at our disposal, we can generally determine at
our own discretion ... the whole of the circumstances which shall
be present along with it; and thus, knowing exactly the simultaneous
state of everything else which is within the reach of _A's_ influence,
we have only to observe what alteration is made in that state by the
presence of _A_.

For example, by the electric machine we can produce, i