ther. Each plot may thus be regarded as
practically composing the same set of conditions, and any difference
in the product may with reasonable probability be ascribed to the
single difference in the antecedents, the manures which it is desired
to compare.

II.--APPLICATION OF THE PRINCIPLE.

The principle of referring a phenomenon to the only immediately
preceding change in antecedent circumstances that could possibly have
affected it, is so simple and so often employed by everybody every
day, that at first we do not see how there can be any difficulty
about it or any possibility of error. And once we understand how many
difficulties there are in reaching exact knowledge even on this simple
principle, and what care has to be taken, we are apt to overrate its
value, and to imagine that it carries us further than it really does.
The scientific expert must know how to apply this principle, and a
single application of it with the proper precautions may take him
days or weeks, and yet all that can be made good by it may carry but a
little way towards the knowledge of which he is in search.

When the circumstances are simple and the effect follows at once, as
when hot water scalds, or a blow with a stick breaks a pane of glass,
there can be no doubt of the causal connexion so far, though plenty
of room for further inquiry into the why. But the mere succession of
phenomena may be obscure. We may introduce more than one agent
without knowing it, and if some time elapses between the experimental
interference and the appearance of the effect, other agents may come
in without our knowledge.

We must know exactly what it is that we introduce and all the
circumstances into which we introduce it. We are apt to ignore the
presence of antecedents that are really influential in the result. A
man heated by work in the harvest field hastily swallows a glass of
water, and drops down dead. There is no doubt that the drinking of the
water was a causal antecedent, but the influential circumstance
may not have been the quantity or the quality of the liquid but its
temperature, and this was introduced into the situation as well as a
certain amount of the liquid components. In making tea we put in so
much tea and so much boiling water. But the temperature of the pot
is also an influential circumstance in the resulting infusion. So in
chemical experiments, where one might expect the result to depend only
upon the proportions of the ingred