found access in the
course of the interval before life appeared. At a certain stage in
Pouchet's process he had occasion to dip the mouths of the flasks
in mercury. It occurred to Pasteur in repeating the experiments that
germs might have found their way in from the atmospheric dust on the
surface of this mercury. That this was so was rendered probable by his
finding that when he carefully cleansed the surface of the mercury no
life appeared afterwards in his flasks.

The application of the principle in human affairs is rendered
uncertain by the immense complication of the phenomena, the difficulty
of experiment, and the special liability of our judgments to
prejudice. That men and communities of men are influenced by
circumstances is not to be denied, and the influence of circumstances,
if it is to be traced at all, must be traced through observed facts.
Observation of the succession of phenomena must be part at least of
any method of tracing cause and effect. We must watch what follows
upon the addition of new agencies to a previously existing sum. But
we can seldom or never get a decisive observation from one pair of
instances, a clear case of difference of result preceded by a single
difference in the antecedents. The simple Method of Experimental
Addition or Subtraction is practically inapplicable. We can do nothing
with a man analogous to putting him into a hermetically sealed retort.
Any man or any community that is the subject of our observations
must be under manifold influences. Each of them probably works some
fraction of the total change observable, but how are they to be
disentangled? Consider, for example, how impossible it would be
to prove in an individual case, on the strict principle of Single
Difference, that Evil communications corrupt good manners. Moral
deterioration may be observed following upon the introduction of
an evil companion, but how can we make sure that no other degrading
influence has operated, and that no original depravity has developed
itself in the interval? Yet such propositions of moral causation can
be proved from experience with reasonable probability. Only it must
be by more extended observations than the strict Method of Difference
takes into account. The method is to observe repeated coincidences
between evil companionship and moral deterioration, and to account for
this in accordance with still wider observations of the interaction of
human personalities.

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