ing
conclusions about causation, they are guides to the analysis of
observations and to the preparation of experiments. To many eminent
investigators the Canons (as such) have been unknown; but they prepared
their work effectively so far only as they had definite ideas to the
same purport. A definite conception of the conditions of proof is the
necessary antecedent of whatever preparations may be made for proving
anything.

CHAPTER XVI

THE CANONS OF DIRECT INDUCTION

Sec. 1. Let me begin by borrowing an example from Bain (_Logic_: B. III. c.
6). The North-East wind is generally detested in this country: as long
as it blows few people feel at their best. Occasional well-known causes
of a wind being injurious are violence, excessive heat or cold,
excessive dryness or moisture, electrical condition, the being laden
with dust or exhalations. Let the hypothesis be that the last is the
cause of the North-East wind's unwholesome quality; since we know it is
a ground current setting from the pole toward the equator and bent
westward by the rotation of the earth; so that, reaching us over
thousands of miles of land, it may well be fraught with dust, effluvia,
and microbes. Now, examining many cases of North-East wind, we find that
this is the only circumstance in which all the instances agree: for it
is sometimes cold, sometimes hot; generally dry, but sometimes wet;
sometimes light, sometimes violent, and of all electrical conditions.
Each of the other circumstances, then, can be omitted without the N.E.
wind ceasing to be noxious; but one circumstance is never absent,
namely, that it is a ground current. That circumstance, therefore, is
probably the cause of its injuriousness. This case illustrates:--

(I) THE CANON OF AGREEMENT.

_If two or more instances of a phenomenon under investigation have only
one other circumstance (antecedent or consequent) in common, that
circumstance is probably the cause (or an indispensable condition) or
the effect of the phenomenon, or is connected with it by causation._

This rule of proof (so far as it is used to establish direct causation)
depends, first, upon observation of an invariable connection between the
given phenomenon and one other circumstance; and, secondly, upon I. (a)
and II. (b) among the propositions obtained from the unconditionality of
causation at the close of the last chapter.

To prove that A is causally related to _p_, suppose two instances of the
occ