aries a
single one of its elements. The method has been stated as follows:

If an instance in which the phenomenon occurs, and an instance in which
it does not occur, have every circumstance in common save one, that one
occurring only in the former; the circumstance in which alone the two
instances differ, is the effect, or the cause, or an indispensable part
of the cause, of the phenomenon.[35]

The principle is clearer and more apprehensible in the concrete example
than in the abstract statement; as a matter of fact it is applied in
every experimental search for a cause. The Agricultural College of New
York, for example, in the course of certain experiments on apple
orchards, bought an orchard which had not been yielding well, and
divided it into halves; one half was then kept plowed and cultivated,
the other half was left in grass; otherwise the treatment was the same.
When the half which was kept cultivated gave a much larger yield than
the other, it was safe to infer that the cultivation was the cause of
the heavier yield. Dr. Ehrlich, the great German pathologist, is said to
have tried six hundred and five different substances before he found one
which would kill the germ of a certain disease; in each experiment he
was using the method of difference, keeping the conditions the same in
all except a single point, which was the addition of the substance used
in that particular experiment. Wherever the conditions of an experiment
can be thus controlled, the method of difference gives a very accurate
way of discovering causes. With advancing knowledge a supposed cause may
be in turn analyzed in such a way that each of its parts can be
separately varied, in order to come more closely to the actual sequence
involved.

It has been pointed out[36] that the two methods are really statements
of what is required for the verification of a theory at two stages of
its growth: when we are first getting a glimpse of a causal connection
between two facts we collect all the cases in which they occur in as
much variety as possible, to see if the connection is really universal;
then, having established the universal sequence, we come to close
quarters with it in a single critical instance, varying the conditions
singly until we run down the one without which the effect cannot take
place.

No neater and more illuminating example of this relation between the two
methods and the successful working of them can be found than that in th